diff --git a/modules/openapi-generator/src/main/java/org/openapitools/codegen/utils/ModelUtils.java b/modules/openapi-generator/src/main/java/org/openapitools/codegen/utils/ModelUtils.java
index 6beb9a7616f..8ac76f74734 100644
--- a/modules/openapi-generator/src/main/java/org/openapitools/codegen/utils/ModelUtils.java
+++ b/modules/openapi-generator/src/main/java/org/openapitools/codegen/utils/ModelUtils.java
@@ -547,20 +547,17 @@ public class ModelUtils {
         if ("object".equals(schema.getType())) {
             // no properties
             if ((schema.getProperties() == null || schema.getProperties().isEmpty())) {
-                if (schema.getAdditionalProperties() == null) {
+                Schema addlProps = getAdditionalProperties(schema);
+                // additionalProperties not defined
+                if (addlProps == null) {
                     return true;
                 } else {
-                    // additionalProperties set to true
-                    if (schema.getAdditionalProperties() instanceof Boolean
-                            && (Boolean) schema.getAdditionalProperties()) {
-                        return true;
-                    }
-
-                    // additionalProperties is set to {}
-                    if (schema.getAdditionalProperties() instanceof Schema && schema.getAdditionalProperties() != null
-                            && schema.getAdditionalProperties() instanceof ObjectSchema
-                            && ((Schema) schema.getAdditionalProperties()).getProperties().isEmpty()) {
-                        return true;
+                    if (addlProps instanceof ObjectSchema) {
+                        ObjectSchema objSchema = (ObjectSchema) addlProps;
+                        // additionalProperties defined as {}
+                        if (objSchema.getProperties() == null || objSchema.getProperties().isEmpty()) {
+                            return true;
+                        }
                     }
                 }
             }
diff --git a/modules/openapi-generator/src/test/java/org/openapitools/codegen/utils/ModelUtilsTest.java b/modules/openapi-generator/src/test/java/org/openapitools/codegen/utils/ModelUtilsTest.java
index e024897e231..16951a91b5d 100644
--- a/modules/openapi-generator/src/test/java/org/openapitools/codegen/utils/ModelUtilsTest.java
+++ b/modules/openapi-generator/src/test/java/org/openapitools/codegen/utils/ModelUtilsTest.java
@@ -199,4 +199,32 @@ public class ModelUtilsTest {
 
         Assert.assertEquals(refToComposedSchema, ModelUtils.unaliasSchema(allSchemas, refToComposedSchema));
     }
+
+    /**
+     * Issue https://github.com/OpenAPITools/openapi-generator/issues/1624.
+     * ModelUtils.isFreeFormObject() should not throw an NPE when passed an empty
+     * object schema that has additionalProperties defined as an empty object schema.
+     */
+    @Test
+    public void testIsFreeFormObject() {
+        // Create initial "empty" object schema.
+        ObjectSchema objSchema = new ObjectSchema();
+        Assert.assertTrue(ModelUtils.isFreeFormObject(objSchema));
+
+        // Set additionalProperties to an empty ObjectSchema.
+        objSchema.setAdditionalProperties(new ObjectSchema());
+        Assert.assertTrue(ModelUtils.isFreeFormObject(objSchema));
+
+        // Add a single property to the schema (no longer a free-form object).
+        Map<String, Schema> props = new HashMap<>();
+        props.put("prop1", new StringSchema());
+        objSchema.setProperties(props);
+        Assert.assertFalse(ModelUtils.isFreeFormObject(objSchema));
+
+        // Test a non-object schema
+        Assert.assertFalse(ModelUtils.isFreeFormObject(new StringSchema()));
+
+        // Test a null schema
+        Assert.assertFalse(ModelUtils.isFreeFormObject(null));
+    }
 }