diff --git a/modules/openapi-generator/src/main/resources/kotlin-client/libraries/jvm-okhttp/api.mustache b/modules/openapi-generator/src/main/resources/kotlin-client/libraries/jvm-okhttp/api.mustache
index ee9bf10a764..0bcf5c9ba84 100644
--- a/modules/openapi-generator/src/main/resources/kotlin-client/libraries/jvm-okhttp/api.mustache
+++ b/modules/openapi-generator/src/main/resources/kotlin-client/libraries/jvm-okhttp/api.mustache
@@ -64,7 +64,10 @@ import {{packageName}}.infrastructure.toMultiValue
                 {{/queryParams}}
             }
             {{/hasQueryParams}}
-        val localVariableHeaders: MutableMap<String, String> = mutableMapOf({{#hasFormParams}}"Content-Type" to {{^consumes}}"multipart/form-data"{{/consumes}}{{#consumes.0}}"{{{mediaType}}}"{{/consumes.0}}{{/hasFormParams}}{{^hasHeaderParams}}){{/hasHeaderParams}}{{#hasHeaderParams}}{{#hasFormParams}}, {{/hasFormParams}}{{#headerParams}}"{{baseName}}" to {{#isContainer}}{{{paramName}}}.joinToString(separator = collectionDelimiter("{{collectionFormat}}")){{/isContainer}}{{^isContainer}}{{{paramName}}}.toString(){{/isContainer}}{{#hasMore}}, {{/hasMore}}{{/headerParams}}){{/hasHeaderParams}}
+        val localVariableHeaders: MutableMap<String, String> = mutableMapOf({{#hasFormParams}}"Content-Type" to {{^consumes}}"multipart/form-data"{{/consumes}}{{#consumes.0}}"{{{mediaType}}}"{{/consumes.0}}{{/hasFormParams}})
+        {{#headerParams}}
+        {{{paramName}}}?.apply { localVariableHeaders["{{baseName}}"] = {{#isContainer}}this.joinToString(separator = collectionDelimiter("{{collectionFormat}}")){{/isContainer}}{{^isContainer}}this.toString(){{/isContainer}} }
+        {{/headerParams}}
         val localVariableConfig = RequestConfig(
             RequestMethod.{{httpMethod}},
             "{{path}}"{{#pathParams}}.replace("{"+"{{baseName}}"+"}", "${{{paramName}}}"){{/pathParams}},
diff --git a/samples/client/petstore/kotlin-gson/src/main/kotlin/org/openapitools/client/apis/PetApi.kt b/samples/client/petstore/kotlin-gson/src/main/kotlin/org/openapitools/client/apis/PetApi.kt
index 60f72b51af9..e4f7f4eeae0 100644
--- a/samples/client/petstore/kotlin-gson/src/main/kotlin/org/openapitools/client/apis/PetApi.kt
+++ b/samples/client/petstore/kotlin-gson/src/main/kotlin/org/openapitools/client/apis/PetApi.kt
@@ -88,7 +88,8 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun deletePet(petId: kotlin.Long, apiKey: kotlin.String?) : Unit {
         val localVariableBody: kotlin.Any? = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
-        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("api_key" to apiKey.toString())
+        val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
+        apiKey?.apply { localVariableHeaders["api_key"] = this.toString() }
         val localVariableConfig = RequestConfig(
             RequestMethod.DELETE,
             "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
diff --git a/samples/client/petstore/kotlin-jackson/src/main/kotlin/org/openapitools/client/apis/PetApi.kt b/samples/client/petstore/kotlin-jackson/src/main/kotlin/org/openapitools/client/apis/PetApi.kt
index 60f72b51af9..e4f7f4eeae0 100644
--- a/samples/client/petstore/kotlin-jackson/src/main/kotlin/org/openapitools/client/apis/PetApi.kt
+++ b/samples/client/petstore/kotlin-jackson/src/main/kotlin/org/openapitools/client/apis/PetApi.kt
@@ -88,7 +88,8 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun deletePet(petId: kotlin.Long, apiKey: kotlin.String?) : Unit {
         val localVariableBody: kotlin.Any? = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
-        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("api_key" to apiKey.toString())
+        val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
+        apiKey?.apply { localVariableHeaders["api_key"] = this.toString() }
         val localVariableConfig = RequestConfig(
             RequestMethod.DELETE,
             "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
diff --git a/samples/client/petstore/kotlin-json-request-string/src/main/kotlin/org/openapitools/client/apis/PetApi.kt b/samples/client/petstore/kotlin-json-request-string/src/main/kotlin/org/openapitools/client/apis/PetApi.kt
index 584c6ef0686..6685f0a73d4 100644
--- a/samples/client/petstore/kotlin-json-request-string/src/main/kotlin/org/openapitools/client/apis/PetApi.kt
+++ b/samples/client/petstore/kotlin-json-request-string/src/main/kotlin/org/openapitools/client/apis/PetApi.kt
@@ -88,7 +88,8 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun deletePet(petId: kotlin.Long, apiKey: kotlin.String?) : Unit {
         val localVariableBody: kotlin.Any? = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
-        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("api_key" to apiKey.toString())
+        val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
+        apiKey?.apply { localVariableHeaders["api_key"] = this.toString() }
         val localVariableConfig = RequestConfig(
             RequestMethod.DELETE,
             "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
diff --git a/samples/client/petstore/kotlin-jvm-okhttp4-coroutines/src/main/kotlin/org/openapitools/client/apis/PetApi.kt b/samples/client/petstore/kotlin-jvm-okhttp4-coroutines/src/main/kotlin/org/openapitools/client/apis/PetApi.kt
index 1cff24913d6..1ca2057e217 100644
--- a/samples/client/petstore/kotlin-jvm-okhttp4-coroutines/src/main/kotlin/org/openapitools/client/apis/PetApi.kt
+++ b/samples/client/petstore/kotlin-jvm-okhttp4-coroutines/src/main/kotlin/org/openapitools/client/apis/PetApi.kt
@@ -88,7 +88,8 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     suspend fun deletePet(petId: kotlin.Long, apiKey: kotlin.String?) : Unit {
         val localVariableBody: kotlin.Any? = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
-        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("api_key" to apiKey.toString())
+        val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
+        apiKey?.apply { localVariableHeaders["api_key"] = this.toString() }
         val localVariableConfig = RequestConfig(
             RequestMethod.DELETE,
             "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
diff --git a/samples/client/petstore/kotlin-moshi-codegen/src/main/kotlin/org/openapitools/client/apis/PetApi.kt b/samples/client/petstore/kotlin-moshi-codegen/src/main/kotlin/org/openapitools/client/apis/PetApi.kt
index 60f72b51af9..e4f7f4eeae0 100644
--- a/samples/client/petstore/kotlin-moshi-codegen/src/main/kotlin/org/openapitools/client/apis/PetApi.kt
+++ b/samples/client/petstore/kotlin-moshi-codegen/src/main/kotlin/org/openapitools/client/apis/PetApi.kt
@@ -88,7 +88,8 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun deletePet(petId: kotlin.Long, apiKey: kotlin.String?) : Unit {
         val localVariableBody: kotlin.Any? = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
-        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("api_key" to apiKey.toString())
+        val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
+        apiKey?.apply { localVariableHeaders["api_key"] = this.toString() }
         val localVariableConfig = RequestConfig(
             RequestMethod.DELETE,
             "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
diff --git a/samples/client/petstore/kotlin-nonpublic/src/main/kotlin/org/openapitools/client/apis/PetApi.kt b/samples/client/petstore/kotlin-nonpublic/src/main/kotlin/org/openapitools/client/apis/PetApi.kt
index 5e930e96395..69cb6b35ef2 100644
--- a/samples/client/petstore/kotlin-nonpublic/src/main/kotlin/org/openapitools/client/apis/PetApi.kt
+++ b/samples/client/petstore/kotlin-nonpublic/src/main/kotlin/org/openapitools/client/apis/PetApi.kt
@@ -88,7 +88,8 @@ internal class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(bas
     fun deletePet(petId: kotlin.Long, apiKey: kotlin.String?) : Unit {
         val localVariableBody: kotlin.Any? = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
-        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("api_key" to apiKey.toString())
+        val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
+        apiKey?.apply { localVariableHeaders["api_key"] = this.toString() }
         val localVariableConfig = RequestConfig(
             RequestMethod.DELETE,
             "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
diff --git a/samples/client/petstore/kotlin-nullable/src/main/kotlin/org/openapitools/client/apis/PetApi.kt b/samples/client/petstore/kotlin-nullable/src/main/kotlin/org/openapitools/client/apis/PetApi.kt
index 4b9faa2a2c4..ef80c520fd2 100644
--- a/samples/client/petstore/kotlin-nullable/src/main/kotlin/org/openapitools/client/apis/PetApi.kt
+++ b/samples/client/petstore/kotlin-nullable/src/main/kotlin/org/openapitools/client/apis/PetApi.kt
@@ -88,7 +88,8 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun deletePet(petId: kotlin.Long, apiKey: kotlin.String?) : Unit {
         val localVariableBody: kotlin.Any? = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
-        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("api_key" to apiKey.toString())
+        val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
+        apiKey?.apply { localVariableHeaders["api_key"] = this.toString() }
         val localVariableConfig = RequestConfig(
             RequestMethod.DELETE,
             "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
diff --git a/samples/client/petstore/kotlin-okhttp3/src/main/kotlin/org/openapitools/client/apis/PetApi.kt b/samples/client/petstore/kotlin-okhttp3/src/main/kotlin/org/openapitools/client/apis/PetApi.kt
index 60f72b51af9..e4f7f4eeae0 100644
--- a/samples/client/petstore/kotlin-okhttp3/src/main/kotlin/org/openapitools/client/apis/PetApi.kt
+++ b/samples/client/petstore/kotlin-okhttp3/src/main/kotlin/org/openapitools/client/apis/PetApi.kt
@@ -88,7 +88,8 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun deletePet(petId: kotlin.Long, apiKey: kotlin.String?) : Unit {
         val localVariableBody: kotlin.Any? = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
-        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("api_key" to apiKey.toString())
+        val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
+        apiKey?.apply { localVariableHeaders["api_key"] = this.toString() }
         val localVariableConfig = RequestConfig(
             RequestMethod.DELETE,
             "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
diff --git a/samples/client/petstore/kotlin-string/src/main/kotlin/org/openapitools/client/apis/PetApi.kt b/samples/client/petstore/kotlin-string/src/main/kotlin/org/openapitools/client/apis/PetApi.kt
index c95dd04f915..da9784198d1 100644
--- a/samples/client/petstore/kotlin-string/src/main/kotlin/org/openapitools/client/apis/PetApi.kt
+++ b/samples/client/petstore/kotlin-string/src/main/kotlin/org/openapitools/client/apis/PetApi.kt
@@ -88,7 +88,8 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun deletePet(apiKey: kotlin.String?, petId: kotlin.Long) : Unit {
         val localVariableBody: kotlin.Any? = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
-        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("api_key" to apiKey.toString())
+        val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
+        apiKey?.apply { localVariableHeaders["api_key"] = this.toString() }
         val localVariableConfig = RequestConfig(
             RequestMethod.DELETE,
             "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
diff --git a/samples/client/petstore/kotlin-threetenbp/src/main/kotlin/org/openapitools/client/apis/PetApi.kt b/samples/client/petstore/kotlin-threetenbp/src/main/kotlin/org/openapitools/client/apis/PetApi.kt
index 60f72b51af9..e4f7f4eeae0 100644
--- a/samples/client/petstore/kotlin-threetenbp/src/main/kotlin/org/openapitools/client/apis/PetApi.kt
+++ b/samples/client/petstore/kotlin-threetenbp/src/main/kotlin/org/openapitools/client/apis/PetApi.kt
@@ -88,7 +88,8 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun deletePet(petId: kotlin.Long, apiKey: kotlin.String?) : Unit {
         val localVariableBody: kotlin.Any? = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
-        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("api_key" to apiKey.toString())
+        val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
+        apiKey?.apply { localVariableHeaders["api_key"] = this.toString() }
         val localVariableConfig = RequestConfig(
             RequestMethod.DELETE,
             "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
diff --git a/samples/client/petstore/kotlin/src/main/kotlin/org/openapitools/client/apis/PetApi.kt b/samples/client/petstore/kotlin/src/main/kotlin/org/openapitools/client/apis/PetApi.kt
index 60f72b51af9..e4f7f4eeae0 100644
--- a/samples/client/petstore/kotlin/src/main/kotlin/org/openapitools/client/apis/PetApi.kt
+++ b/samples/client/petstore/kotlin/src/main/kotlin/org/openapitools/client/apis/PetApi.kt
@@ -88,7 +88,8 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun deletePet(petId: kotlin.Long, apiKey: kotlin.String?) : Unit {
         val localVariableBody: kotlin.Any? = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
-        val localVariableHeaders: MutableMap<String, String> = mutableMapOf("api_key" to apiKey.toString())
+        val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
+        apiKey?.apply { localVariableHeaders["api_key"] = this.toString() }
         val localVariableConfig = RequestConfig(
             RequestMethod.DELETE,
             "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),