diff --git a/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/PostmanCollectionCodegen.java b/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/PostmanCollectionCodegen.java
index 104ca898608..700e41a6d4f 100644
--- a/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/PostmanCollectionCodegen.java
+++ b/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/PostmanCollectionCodegen.java
@@ -8,6 +8,7 @@ import com.fasterxml.jackson.databind.node.ObjectNode;
 import io.swagger.v3.core.util.Json;
 import io.swagger.v3.oas.models.OpenAPI;
 import io.swagger.v3.oas.models.examples.Example;
+import io.swagger.v3.oas.models.media.Schema;
 import io.swagger.v3.oas.models.servers.ServerVariable;
 import org.openapitools.codegen.*;
 import org.openapitools.codegen.meta.GeneratorMetadata;
@@ -462,6 +463,24 @@ public class PostmanCollectionCodegen extends DefaultCodegen implements CodegenC
         return input;
     }
 
+    /**
+     * Return the default value of the property
+     * <p>
+     * Return null when the default is not defined
+     *
+     * @param schema Property schema
+     * @return string presentation of the default value of the property
+     */
+    @SuppressWarnings("static-method")
+    @Override
+    public String toDefaultValue(Schema schema) {
+        if (schema.getDefault() != null) {
+            return schema.getDefault().toString();
+        }
+
+        return null;
+    }
+
     /**
      * Escape single and/or double quote to avoid code injection
      *
diff --git a/modules/openapi-generator/src/test/java/org/openapitools/codegen/postman/PostmanCollectionCodegenTest.java b/modules/openapi-generator/src/test/java/org/openapitools/codegen/postman/PostmanCollectionCodegenTest.java
index 62bfa5f52d9..fa7225826f8 100644
--- a/modules/openapi-generator/src/test/java/org/openapitools/codegen/postman/PostmanCollectionCodegenTest.java
+++ b/modules/openapi-generator/src/test/java/org/openapitools/codegen/postman/PostmanCollectionCodegenTest.java
@@ -485,7 +485,10 @@ public class PostmanCollectionCodegenTest {
         TestUtils.assertFileExists(path);
         TestUtils.assertFileContains(path, "{ \"key\": \"Content-Type\", \"value\": \"application/json\"");
         TestUtils.assertFileContains(path, "{ \"key\": \"Accept\", \"value\": \"application/json\"");
-        TestUtils.assertFileContains(path, "{ \"key\": \"Custom-Header\", \"value\": \"null\"");
+        // header without default value
+        TestUtils.assertFileContains(path, "{ \"key\": \"Custom-Header\", \"value\": \"\"");
+        // header with default value
+        TestUtils.assertFileContains(path, "{ \"key\": \"Another-Custom-Header\", \"value\": \"abc\"");
     }
 
 
diff --git a/modules/openapi-generator/src/test/resources/3_0/postman-collection/SampleProject.yaml b/modules/openapi-generator/src/test/resources/3_0/postman-collection/SampleProject.yaml
index 68880c12bc1..97786ebeeb4 100644
--- a/modules/openapi-generator/src/test/resources/3_0/postman-collection/SampleProject.yaml
+++ b/modules/openapi-generator/src/test/resources/3_0/postman-collection/SampleProject.yaml
@@ -52,6 +52,12 @@ paths:
           in: header
           schema:
             type: string
+        - description: Custom HTTP header with default
+          name: Another-Custom-Header
+          in: header
+          schema:
+            type: string
+            default: abc
 
       responses:
         '200':
diff --git a/samples/client/petstore/csharp/OpenAPIClient-generichost-net6.0-nrt/src/Org.OpenAPITools/Api/FakeApi.cs b/samples/client/petstore/csharp/OpenAPIClient-generichost-net6.0-nrt/src/Org.OpenAPITools/Api/FakeApi.cs
index 7868e9af673..a7044ed145e 100644
--- a/samples/client/petstore/csharp/OpenAPIClient-generichost-net6.0-nrt/src/Org.OpenAPITools/Api/FakeApi.cs
+++ b/samples/client/petstore/csharp/OpenAPIClient-generichost-net6.0-nrt/src/Org.OpenAPITools/Api/FakeApi.cs
@@ -2023,15 +2023,20 @@ namespace Org.OpenAPITools.Api
         /// <summary>
         /// Validates the request parameters
         /// </summary>
+        /// <param name="enumHeaderString"></param>
         /// <param name="enumHeaderStringArray"></param>
         /// <param name="enumQueryStringArray"></param>
         /// <param name="enumFormStringArray"></param>
         /// <param name="enumHeaderString"></param>
         /// <param name="enumQueryString"></param>
         /// <param name="enumFormString"></param>
+        /// <param name="enumQueryString"></param>
         /// <returns></returns>
         private void ValidateTestEnumParameters(Option<List<string>> enumHeaderStringArray, Option<List<string>> enumQueryStringArray, Option<List<string>> enumFormStringArray, Option<string> enumHeaderString, Option<string> enumQueryString, Option<string> enumFormString)
         {
+            if (enumHeaderString.IsSet && enumHeaderString.Value == null)
+                throw new ArgumentNullException(nameof(enumHeaderString));
+
             if (enumHeaderStringArray.IsSet && enumHeaderStringArray.Value == null)
                 throw new ArgumentNullException(nameof(enumHeaderStringArray));
 
@@ -2049,6 +2054,9 @@ namespace Org.OpenAPITools.Api
 
             if (enumFormString.IsSet && enumFormString.Value == null)
                 throw new ArgumentNullException(nameof(enumFormString));
+
+            if (enumQueryString.IsSet && enumQueryString.Value == null)
+                throw new ArgumentNullException(nameof(enumQueryString));
         }
 
         /// <summary>
diff --git a/samples/schema/postman-collection/postman.json b/samples/schema/postman-collection/postman.json
index 3e411b1726e..a0951aaa1c7 100644
--- a/samples/schema/postman-collection/postman.json
+++ b/samples/schema/postman-collection/postman.json
@@ -35,7 +35,7 @@
                                         },
                                         {
                                             "key": "strCode2",
-                                            "value": "null"
+                                            "value": ""
                                         }
                                     ],
                                     "body": {
@@ -95,7 +95,7 @@
                                         },
                                         {
                                             "key": "strCode2",
-                                            "value": "null"
+                                            "value": ""
                                         }
                                     ],
                                     "body": {
@@ -151,7 +151,11 @@
                                         },
                                         {
                                             "key": "Custom-Header",
-                                            "value": "null"
+                                            "value": ""
+                                        },
+                                        {
+                                            "key": "Another-Custom-Header",
+                                            "value": "abc"
                                         }
                                     ],
                                     "body": {