diff --git a/modules/openapi-generator/src/main/resources/kotlin-client/libraries/jvm-retrofit2/headerParams.mustache b/modules/openapi-generator/src/main/resources/kotlin-client/libraries/jvm-retrofit2/headerParams.mustache
index aa5a8e34673205ce7db2f52f43d347acd0fb4d73..f546244307bd683fa891536adf0fe2ea7396ed4f 100644
--- a/modules/openapi-generator/src/main/resources/kotlin-client/libraries/jvm-retrofit2/headerParams.mustache
+++ b/modules/openapi-generator/src/main/resources/kotlin-client/libraries/jvm-retrofit2/headerParams.mustache
@@ -1 +1 @@
-{{#isHeaderParam}}@Header("{{baseName}}") {{{paramName}}}: {{{dataType}}}{{/isHeaderParam}}
\ No newline at end of file
+{{#isHeaderParam}}@Header("{{baseName}}") {{{paramName}}}: {{{dataType}}}{{#required}}{{#defaultValue}} = {{{defaultValue}}}{{/defaultValue}}{{/required}}{{^required}}?{{#defaultValue}} = {{{defaultValue}}}{{/defaultValue}}{{^defaultValue}} = null{{/defaultValue}}{{/required}}{{/isHeaderParam}}
\ No newline at end of file
diff --git a/samples/client/petstore/kotlin-retrofit2-kotlinx_serialization/src/main/kotlin/org/openapitools/client/apis/PetApi.kt b/samples/client/petstore/kotlin-retrofit2-kotlinx_serialization/src/main/kotlin/org/openapitools/client/apis/PetApi.kt
index e8b980bc87277fde0d2f3166817a18f115333fce..2144090e5c2a2dfcd0dd696c9b4f81753200863f 100644
--- a/samples/client/petstore/kotlin-retrofit2-kotlinx_serialization/src/main/kotlin/org/openapitools/client/apis/PetApi.kt
+++ b/samples/client/petstore/kotlin-retrofit2-kotlinx_serialization/src/main/kotlin/org/openapitools/client/apis/PetApi.kt
@@ -34,7 +34,7 @@ interface PetApi {
      * @return [Call]<[Unit]>
      */
     @DELETE("pet/{petId}")
-    fun deletePet(@Path("petId") petId: kotlin.Long, @Header("api_key") apiKey: kotlin.String): Call<Unit>
+    fun deletePet(@Path("petId") petId: kotlin.Long, @Header("api_key") apiKey: kotlin.String? = null): Call<Unit>
 
     /**
      * Finds Pets by status
diff --git a/samples/client/petstore/kotlin-retrofit2-rx3/src/main/kotlin/org/openapitools/client/apis/PetApi.kt b/samples/client/petstore/kotlin-retrofit2-rx3/src/main/kotlin/org/openapitools/client/apis/PetApi.kt
index 90e791f37afad1f32fe172b24c8477dd9e961dc3..8289e47e328dc9976ed912f0cdda3fe69e5f00f8 100644
--- a/samples/client/petstore/kotlin-retrofit2-rx3/src/main/kotlin/org/openapitools/client/apis/PetApi.kt
+++ b/samples/client/petstore/kotlin-retrofit2-rx3/src/main/kotlin/org/openapitools/client/apis/PetApi.kt
@@ -35,7 +35,7 @@ interface PetApi {
      * @return [Call]<[Unit]>
      */
     @DELETE("pet/{petId}")
-    fun deletePet(@Path("petId") petId: kotlin.Long, @Header("api_key") apiKey: kotlin.String): Completable
+    fun deletePet(@Path("petId") petId: kotlin.Long, @Header("api_key") apiKey: kotlin.String? = null): Completable
 
     /**
      * Finds Pets by status
diff --git a/samples/client/petstore/kotlin-retrofit2/src/main/kotlin/org/openapitools/client/apis/PetApi.kt b/samples/client/petstore/kotlin-retrofit2/src/main/kotlin/org/openapitools/client/apis/PetApi.kt
index e8b980bc87277fde0d2f3166817a18f115333fce..2144090e5c2a2dfcd0dd696c9b4f81753200863f 100644
--- a/samples/client/petstore/kotlin-retrofit2/src/main/kotlin/org/openapitools/client/apis/PetApi.kt
+++ b/samples/client/petstore/kotlin-retrofit2/src/main/kotlin/org/openapitools/client/apis/PetApi.kt
@@ -34,7 +34,7 @@ interface PetApi {
      * @return [Call]<[Unit]>
      */
     @DELETE("pet/{petId}")
-    fun deletePet(@Path("petId") petId: kotlin.Long, @Header("api_key") apiKey: kotlin.String): Call<Unit>
+    fun deletePet(@Path("petId") petId: kotlin.Long, @Header("api_key") apiKey: kotlin.String? = null): Call<Unit>
 
     /**
      * Finds Pets by status