diff --git a/modules/openapi-generator/src/main/resources/Java/libraries/okhttp-gson-nextgen/anyof_model.mustache b/modules/openapi-generator/src/main/resources/Java/libraries/okhttp-gson-nextgen/anyof_model.mustache
index a4c5c86ca2d..04f8a08934c 100644
--- a/modules/openapi-generator/src/main/resources/Java/libraries/okhttp-gson-nextgen/anyof_model.mustache
+++ b/modules/openapi-generator/src/main/resources/Java/libraries/okhttp-gson-nextgen/anyof_model.mustache
@@ -50,6 +50,11 @@ public class {{classname}} extends AbstractOpenApiSchema{{#vendorExtensions.x-im
             return (TypeAdapter<T>) new TypeAdapter<{{classname}}>() {
                 @Override
                 public void write(JsonWriter out, {{classname}} value) throws IOException {
+                    if (value == null || value.getActualInstance() == null) {
+                        elementAdapter.write(out, null);
+                        return;
+                    }
+
                     {{#anyOf}}
                     // check if the actual instance is of the type `{{.}}`
                     if (value.getActualInstance() instanceof {{.}}) {
diff --git a/modules/openapi-generator/src/main/resources/Java/libraries/okhttp-gson-nextgen/oneof_model.mustache b/modules/openapi-generator/src/main/resources/Java/libraries/okhttp-gson-nextgen/oneof_model.mustache
index 0b7c8b6369e..84c4846dc50 100644
--- a/modules/openapi-generator/src/main/resources/Java/libraries/okhttp-gson-nextgen/oneof_model.mustache
+++ b/modules/openapi-generator/src/main/resources/Java/libraries/okhttp-gson-nextgen/oneof_model.mustache
@@ -50,6 +50,11 @@ public class {{classname}} extends AbstractOpenApiSchema{{#vendorExtensions.x-im
             return (TypeAdapter<T>) new TypeAdapter<{{classname}}>() {
                 @Override
                 public void write(JsonWriter out, {{classname}} value) throws IOException {
+                    if (value == null || value.getActualInstance() == null) {
+                        elementAdapter.write(out, null);
+                        return;
+                    }
+
                     {{#oneOf}}
                     // check if the actual instance is of the type `{{.}}`
                     if (value.getActualInstance() instanceof {{.}}) {
diff --git a/samples/client/petstore/java/okhttp-gson-nextgen/src/main/java/org/openapitools/client/model/Fruit.java b/samples/client/petstore/java/okhttp-gson-nextgen/src/main/java/org/openapitools/client/model/Fruit.java
index 6f907a92f0a..4b162d79aec 100644
--- a/samples/client/petstore/java/okhttp-gson-nextgen/src/main/java/org/openapitools/client/model/Fruit.java
+++ b/samples/client/petstore/java/okhttp-gson-nextgen/src/main/java/org/openapitools/client/model/Fruit.java
@@ -78,6 +78,11 @@ public class Fruit extends AbstractOpenApiSchema {
             return (TypeAdapter<T>) new TypeAdapter<Fruit>() {
                 @Override
                 public void write(JsonWriter out, Fruit value) throws IOException {
+                    if (value == null || value.getActualInstance() == null) {
+                        elementAdapter.write(out, null);
+                        return;
+                    }
+
                     // check if the actual instance is of the type `Apple`
                     if (value.getActualInstance() instanceof Apple) {
                         JsonObject obj = adapterApple.toJsonTree((Apple)value.getActualInstance()).getAsJsonObject();
diff --git a/samples/client/petstore/java/okhttp-gson-nextgen/src/main/java/org/openapitools/client/model/FruitReq.java b/samples/client/petstore/java/okhttp-gson-nextgen/src/main/java/org/openapitools/client/model/FruitReq.java
index 303049b6007..13436b1d6e3 100644
--- a/samples/client/petstore/java/okhttp-gson-nextgen/src/main/java/org/openapitools/client/model/FruitReq.java
+++ b/samples/client/petstore/java/okhttp-gson-nextgen/src/main/java/org/openapitools/client/model/FruitReq.java
@@ -78,6 +78,11 @@ public class FruitReq extends AbstractOpenApiSchema {
             return (TypeAdapter<T>) new TypeAdapter<FruitReq>() {
                 @Override
                 public void write(JsonWriter out, FruitReq value) throws IOException {
+                    if (value == null || value.getActualInstance() == null) {
+                        elementAdapter.write(out, null);
+                        return;
+                    }
+
                     // check if the actual instance is of the type `AppleReq`
                     if (value.getActualInstance() instanceof AppleReq) {
                         JsonObject obj = adapterAppleReq.toJsonTree((AppleReq)value.getActualInstance()).getAsJsonObject();
diff --git a/samples/client/petstore/java/okhttp-gson-nextgen/src/main/java/org/openapitools/client/model/GmFruit.java b/samples/client/petstore/java/okhttp-gson-nextgen/src/main/java/org/openapitools/client/model/GmFruit.java
index 441f0330de6..1d07fdfd6ba 100644
--- a/samples/client/petstore/java/okhttp-gson-nextgen/src/main/java/org/openapitools/client/model/GmFruit.java
+++ b/samples/client/petstore/java/okhttp-gson-nextgen/src/main/java/org/openapitools/client/model/GmFruit.java
@@ -78,6 +78,11 @@ public class GmFruit extends AbstractOpenApiSchema {
             return (TypeAdapter<T>) new TypeAdapter<GmFruit>() {
                 @Override
                 public void write(JsonWriter out, GmFruit value) throws IOException {
+                    if (value == null || value.getActualInstance() == null) {
+                        elementAdapter.write(out, null);
+                        return;
+                    }
+
                     // check if the actual instance is of the type `Apple`
                     if (value.getActualInstance() instanceof Apple) {
                         JsonObject obj = adapterApple.toJsonTree((Apple)value.getActualInstance()).getAsJsonObject();
diff --git a/samples/client/petstore/java/okhttp-gson-nextgen/src/main/java/org/openapitools/client/model/Mammal.java b/samples/client/petstore/java/okhttp-gson-nextgen/src/main/java/org/openapitools/client/model/Mammal.java
index 466644f4b7c..fcd1935126b 100644
--- a/samples/client/petstore/java/okhttp-gson-nextgen/src/main/java/org/openapitools/client/model/Mammal.java
+++ b/samples/client/petstore/java/okhttp-gson-nextgen/src/main/java/org/openapitools/client/model/Mammal.java
@@ -79,6 +79,11 @@ public class Mammal extends AbstractOpenApiSchema {
             return (TypeAdapter<T>) new TypeAdapter<Mammal>() {
                 @Override
                 public void write(JsonWriter out, Mammal value) throws IOException {
+                    if (value == null || value.getActualInstance() == null) {
+                        elementAdapter.write(out, null);
+                        return;
+                    }
+
                     // check if the actual instance is of the type `Pig`
                     if (value.getActualInstance() instanceof Pig) {
                         JsonObject obj = adapterPig.toJsonTree((Pig)value.getActualInstance()).getAsJsonObject();
diff --git a/samples/client/petstore/java/okhttp-gson-nextgen/src/main/java/org/openapitools/client/model/NullableShape.java b/samples/client/petstore/java/okhttp-gson-nextgen/src/main/java/org/openapitools/client/model/NullableShape.java
index 67b42a30572..bb11f4eb0e9 100644
--- a/samples/client/petstore/java/okhttp-gson-nextgen/src/main/java/org/openapitools/client/model/NullableShape.java
+++ b/samples/client/petstore/java/okhttp-gson-nextgen/src/main/java/org/openapitools/client/model/NullableShape.java
@@ -77,6 +77,11 @@ public class NullableShape extends AbstractOpenApiSchema {
             return (TypeAdapter<T>) new TypeAdapter<NullableShape>() {
                 @Override
                 public void write(JsonWriter out, NullableShape value) throws IOException {
+                    if (value == null || value.getActualInstance() == null) {
+                        elementAdapter.write(out, null);
+                        return;
+                    }
+
                     // check if the actual instance is of the type `Quadrilateral`
                     if (value.getActualInstance() instanceof Quadrilateral) {
                         JsonObject obj = adapterQuadrilateral.toJsonTree((Quadrilateral)value.getActualInstance()).getAsJsonObject();
diff --git a/samples/client/petstore/java/okhttp-gson-nextgen/src/main/java/org/openapitools/client/model/Pig.java b/samples/client/petstore/java/okhttp-gson-nextgen/src/main/java/org/openapitools/client/model/Pig.java
index ea0b58cbf90..9e2015ba672 100644
--- a/samples/client/petstore/java/okhttp-gson-nextgen/src/main/java/org/openapitools/client/model/Pig.java
+++ b/samples/client/petstore/java/okhttp-gson-nextgen/src/main/java/org/openapitools/client/model/Pig.java
@@ -77,6 +77,11 @@ public class Pig extends AbstractOpenApiSchema {
             return (TypeAdapter<T>) new TypeAdapter<Pig>() {
                 @Override
                 public void write(JsonWriter out, Pig value) throws IOException {
+                    if (value == null || value.getActualInstance() == null) {
+                        elementAdapter.write(out, null);
+                        return;
+                    }
+
                     // check if the actual instance is of the type `BasquePig`
                     if (value.getActualInstance() instanceof BasquePig) {
                         JsonObject obj = adapterBasquePig.toJsonTree((BasquePig)value.getActualInstance()).getAsJsonObject();
diff --git a/samples/client/petstore/java/okhttp-gson-nextgen/src/main/java/org/openapitools/client/model/Quadrilateral.java b/samples/client/petstore/java/okhttp-gson-nextgen/src/main/java/org/openapitools/client/model/Quadrilateral.java
index b91dda2eece..04a317679bd 100644
--- a/samples/client/petstore/java/okhttp-gson-nextgen/src/main/java/org/openapitools/client/model/Quadrilateral.java
+++ b/samples/client/petstore/java/okhttp-gson-nextgen/src/main/java/org/openapitools/client/model/Quadrilateral.java
@@ -77,6 +77,11 @@ public class Quadrilateral extends AbstractOpenApiSchema {
             return (TypeAdapter<T>) new TypeAdapter<Quadrilateral>() {
                 @Override
                 public void write(JsonWriter out, Quadrilateral value) throws IOException {
+                    if (value == null || value.getActualInstance() == null) {
+                        elementAdapter.write(out, null);
+                        return;
+                    }
+
                     // check if the actual instance is of the type `ComplexQuadrilateral`
                     if (value.getActualInstance() instanceof ComplexQuadrilateral) {
                         JsonObject obj = adapterComplexQuadrilateral.toJsonTree((ComplexQuadrilateral)value.getActualInstance()).getAsJsonObject();
diff --git a/samples/client/petstore/java/okhttp-gson-nextgen/src/main/java/org/openapitools/client/model/Shape.java b/samples/client/petstore/java/okhttp-gson-nextgen/src/main/java/org/openapitools/client/model/Shape.java
index df7274029b2..babaf6be36f 100644
--- a/samples/client/petstore/java/okhttp-gson-nextgen/src/main/java/org/openapitools/client/model/Shape.java
+++ b/samples/client/petstore/java/okhttp-gson-nextgen/src/main/java/org/openapitools/client/model/Shape.java
@@ -77,6 +77,11 @@ public class Shape extends AbstractOpenApiSchema {
             return (TypeAdapter<T>) new TypeAdapter<Shape>() {
                 @Override
                 public void write(JsonWriter out, Shape value) throws IOException {
+                    if (value == null || value.getActualInstance() == null) {
+                        elementAdapter.write(out, null);
+                        return;
+                    }
+
                     // check if the actual instance is of the type `Quadrilateral`
                     if (value.getActualInstance() instanceof Quadrilateral) {
                         JsonObject obj = adapterQuadrilateral.toJsonTree((Quadrilateral)value.getActualInstance()).getAsJsonObject();
diff --git a/samples/client/petstore/java/okhttp-gson-nextgen/src/main/java/org/openapitools/client/model/ShapeOrNull.java b/samples/client/petstore/java/okhttp-gson-nextgen/src/main/java/org/openapitools/client/model/ShapeOrNull.java
index 7c6f090482a..6b413bce9a3 100644
--- a/samples/client/petstore/java/okhttp-gson-nextgen/src/main/java/org/openapitools/client/model/ShapeOrNull.java
+++ b/samples/client/petstore/java/okhttp-gson-nextgen/src/main/java/org/openapitools/client/model/ShapeOrNull.java
@@ -77,6 +77,11 @@ public class ShapeOrNull extends AbstractOpenApiSchema {
             return (TypeAdapter<T>) new TypeAdapter<ShapeOrNull>() {
                 @Override
                 public void write(JsonWriter out, ShapeOrNull value) throws IOException {
+                    if (value == null || value.getActualInstance() == null) {
+                        elementAdapter.write(out, null);
+                        return;
+                    }
+
                     // check if the actual instance is of the type `Quadrilateral`
                     if (value.getActualInstance() instanceof Quadrilateral) {
                         JsonObject obj = adapterQuadrilateral.toJsonTree((Quadrilateral)value.getActualInstance()).getAsJsonObject();
diff --git a/samples/client/petstore/java/okhttp-gson-nextgen/src/main/java/org/openapitools/client/model/Triangle.java b/samples/client/petstore/java/okhttp-gson-nextgen/src/main/java/org/openapitools/client/model/Triangle.java
index e9e70e6be71..f9a3276d9b1 100644
--- a/samples/client/petstore/java/okhttp-gson-nextgen/src/main/java/org/openapitools/client/model/Triangle.java
+++ b/samples/client/petstore/java/okhttp-gson-nextgen/src/main/java/org/openapitools/client/model/Triangle.java
@@ -79,6 +79,11 @@ public class Triangle extends AbstractOpenApiSchema {
             return (TypeAdapter<T>) new TypeAdapter<Triangle>() {
                 @Override
                 public void write(JsonWriter out, Triangle value) throws IOException {
+                    if (value == null || value.getActualInstance() == null) {
+                        elementAdapter.write(out, null);
+                        return;
+                    }
+
                     // check if the actual instance is of the type `EquilateralTriangle`
                     if (value.getActualInstance() instanceof EquilateralTriangle) {
                         JsonObject obj = adapterEquilateralTriangle.toJsonTree((EquilateralTriangle)value.getActualInstance()).getAsJsonObject();
diff --git a/samples/client/petstore/java/okhttp-gson-nextgen/src/test/java/org/openapitools/client/JSONTest.java b/samples/client/petstore/java/okhttp-gson-nextgen/src/test/java/org/openapitools/client/JSONTest.java
index c6b3c2345dd..4c3fbb12ec5 100644
--- a/samples/client/petstore/java/okhttp-gson-nextgen/src/test/java/org/openapitools/client/JSONTest.java
+++ b/samples/client/petstore/java/okhttp-gson-nextgen/src/test/java/org/openapitools/client/JSONTest.java
@@ -314,6 +314,13 @@ public class JSONTest {
             assertEquals(inst2.getMealy(), false);
             assertEquals(json.getGson().toJson(inst2), "{\"cultivar\":\"golden delicious\",\"mealy\":false}");
         }
+        {
+            // test to ensure the oneOf object can be serialized to "null" correctly
+            FruitReq o = new FruitReq();
+            assertEquals(o.getActualInstance(), null);
+            assertEquals(json.getGson().toJson(o), "null");
+            assertEquals(json.getGson().toJson(null), "null");
+        }
         {
             // Same test, but this time with additional (undeclared) properties.
             // Since FruitReq has additionalProperties: false, deserialization should fail.