diff --git a/modules/openapi-generator/src/main/resources/kotlin-client/api.mustache b/modules/openapi-generator/src/main/resources/kotlin-client/api.mustache
index 0a2b8b83bfe..c86a00b405d 100644
--- a/modules/openapi-generator/src/main/resources/kotlin-client/api.mustache
+++ b/modules/openapi-generator/src/main/resources/kotlin-client/api.mustache
@@ -20,7 +20,7 @@ class {{classname}}(basePath: kotlin.String = "{{{basePath}}}") : ApiClient(base
     {{/allParams}}* @return {{#returnType}}{{{returnType}}}{{/returnType}}{{^returnType}}void{{/returnType}}
     */{{#returnType}}
     @Suppress("UNCHECKED_CAST"){{/returnType}}
-    fun {{operationId}}({{#allParams}}{{paramName}}: {{{dataType}}}{{#hasMore}}, {{/hasMore}}{{/allParams}}) : {{#returnType}}{{{returnType}}}{{/returnType}}{{^returnType}}Unit{{/returnType}} {
+    fun {{operationId}}({{#allParams}}{{paramName}}: {{{dataType}}}{{^required}}?{{/required}}{{#hasMore}}, {{/hasMore}}{{/allParams}}) : {{#returnType}}{{{returnType}}}{{/returnType}}{{^returnType}}Unit{{/returnType}} {
         val localVariableBody: kotlin.Any? = {{#hasBodyParam}}{{#bodyParams}}{{paramName}}{{/bodyParams}}{{/hasBodyParam}}{{^hasBodyParam}}{{^hasFormParams}}null{{/hasFormParams}}{{#hasFormParams}}mapOf({{#formParams}}"{{{baseName}}}" to "${{paramName}}"{{#hasMore}}, {{/hasMore}}{{/formParams}}){{/hasFormParams}}{{/hasBodyParam}}
         val localVariableQuery: MultiValueMap = {{^hasQueryParams}}mapOf(){{/hasQueryParams}}{{#hasQueryParams}}mapOf({{#queryParams}}"{{baseName}}" to {{#isContainer}}toMultiValue({{paramName}}.toList(), "{{collectionFormat}}"){{/isContainer}}{{^isContainer}}listOf("${{paramName}}"){{/isContainer}}{{#hasMore}}, {{/hasMore}}{{/queryParams}}){{/hasQueryParams}}
         val localVariableHeaders: kotlin.collections.Map<kotlin.String,kotlin.String> = mapOf({{#hasFormParams}}"Content-Type" to "multipart/form-data"{{/hasFormParams}}{{^hasHeaderParams}}){{/hasHeaderParams}}{{#hasHeaderParams}}{{#hasFormParams}}, {{/hasFormParams}}{{#headerParams}}"{{baseName}}" to {{#isContainer}}{{paramName}}.joinToString(separator = collectionDelimiter("{{collectionFormat}}")){{/isContainer}}{{^isContainer}}{{paramName}}.toString(){{/isContainer}}{{#hasMore}}, {{/hasMore}}{{/headerParams}}){{/hasHeaderParams}}
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 72d24c820e7..83e8ec2abf0 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
@@ -56,7 +56,7 @@ class PetApi(basePath: kotlin.String = "http://petstore.swagger.io/v2") : ApiCli
     * @param apiKey  (optional, default to null)
     * @return void
     */
-    fun deletePet(petId: kotlin.Long, apiKey: kotlin.String) : Unit {
+    fun deletePet(petId: kotlin.Long, apiKey: kotlin.String?) : Unit {
         val localVariableBody: kotlin.Any? = null
         val localVariableQuery: MultiValueMap = mapOf()
         val localVariableHeaders: kotlin.collections.Map<kotlin.String,kotlin.String> = mapOf("api_key" to apiKey.toString())
@@ -216,7 +216,7 @@ class PetApi(basePath: kotlin.String = "http://petstore.swagger.io/v2") : ApiCli
     * @param status Updated status of the pet (optional, default to null)
     * @return void
     */
-    fun updatePetWithForm(petId: kotlin.Long, name: kotlin.String, status: kotlin.String) : Unit {
+    fun updatePetWithForm(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : Unit {
         val localVariableBody: kotlin.Any? = mapOf("name" to "$name", "status" to "$status")
         val localVariableQuery: MultiValueMap = mapOf()
         val localVariableHeaders: kotlin.collections.Map<kotlin.String,kotlin.String> = mapOf("Content-Type" to "multipart/form-data")
@@ -250,7 +250,7 @@ class PetApi(basePath: kotlin.String = "http://petstore.swagger.io/v2") : ApiCli
     * @return ApiResponse
     */
     @Suppress("UNCHECKED_CAST")
-    fun uploadFile(petId: kotlin.Long, additionalMetadata: kotlin.String, file: java.io.File) : ApiResponse {
+    fun uploadFile(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : ApiResponse {
         val localVariableBody: kotlin.Any? = mapOf("additionalMetadata" to "$additionalMetadata", "file" to "$file")
         val localVariableQuery: MultiValueMap = mapOf()
         val localVariableHeaders: kotlin.collections.Map<kotlin.String,kotlin.String> = mapOf("Content-Type" to "multipart/form-data")
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 e43d959f8aa..9c6df9475da 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
@@ -57,7 +57,7 @@ class PetApi(basePath: kotlin.String = "http://petstore.swagger.io/v2") : ApiCli
     * @param apiKey  (optional, default to null)
     * @return void
     */
-    fun deletePet(petId: kotlin.Long, apiKey: kotlin.String) : Unit {
+    fun deletePet(petId: kotlin.Long, apiKey: kotlin.String?) : Unit {
         val localVariableBody: kotlin.Any? = null
         val localVariableQuery: MultiValueMap = mapOf()
         val localVariableHeaders: kotlin.collections.Map<kotlin.String,kotlin.String> = mapOf("api_key" to apiKey.toString())
@@ -217,7 +217,7 @@ class PetApi(basePath: kotlin.String = "http://petstore.swagger.io/v2") : ApiCli
     * @param status Updated status of the pet (optional, default to null)
     * @return void
     */
-    fun updatePetWithForm(petId: kotlin.Long, name: kotlin.String, status: kotlin.String) : Unit {
+    fun updatePetWithForm(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : Unit {
         val localVariableBody: kotlin.Any? = mapOf("name" to "$name", "status" to "$status")
         val localVariableQuery: MultiValueMap = mapOf()
         val localVariableHeaders: kotlin.collections.Map<kotlin.String,kotlin.String> = mapOf("Content-Type" to "multipart/form-data")
@@ -251,7 +251,7 @@ class PetApi(basePath: kotlin.String = "http://petstore.swagger.io/v2") : ApiCli
     * @return ApiResponse
     */
     @Suppress("UNCHECKED_CAST")
-    fun uploadFile(petId: kotlin.Long, additionalMetadata: kotlin.String, file: java.io.File) : ApiResponse {
+    fun uploadFile(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : ApiResponse {
         val localVariableBody: kotlin.Any? = mapOf("additionalMetadata" to "$additionalMetadata", "file" to "$file")
         val localVariableQuery: MultiValueMap = mapOf()
         val localVariableHeaders: kotlin.collections.Map<kotlin.String,kotlin.String> = mapOf("Content-Type" to "multipart/form-data")
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 72d24c820e7..83e8ec2abf0 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
@@ -56,7 +56,7 @@ class PetApi(basePath: kotlin.String = "http://petstore.swagger.io/v2") : ApiCli
     * @param apiKey  (optional, default to null)
     * @return void
     */
-    fun deletePet(petId: kotlin.Long, apiKey: kotlin.String) : Unit {
+    fun deletePet(petId: kotlin.Long, apiKey: kotlin.String?) : Unit {
         val localVariableBody: kotlin.Any? = null
         val localVariableQuery: MultiValueMap = mapOf()
         val localVariableHeaders: kotlin.collections.Map<kotlin.String,kotlin.String> = mapOf("api_key" to apiKey.toString())
@@ -216,7 +216,7 @@ class PetApi(basePath: kotlin.String = "http://petstore.swagger.io/v2") : ApiCli
     * @param status Updated status of the pet (optional, default to null)
     * @return void
     */
-    fun updatePetWithForm(petId: kotlin.Long, name: kotlin.String, status: kotlin.String) : Unit {
+    fun updatePetWithForm(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : Unit {
         val localVariableBody: kotlin.Any? = mapOf("name" to "$name", "status" to "$status")
         val localVariableQuery: MultiValueMap = mapOf()
         val localVariableHeaders: kotlin.collections.Map<kotlin.String,kotlin.String> = mapOf("Content-Type" to "multipart/form-data")
@@ -250,7 +250,7 @@ class PetApi(basePath: kotlin.String = "http://petstore.swagger.io/v2") : ApiCli
     * @return ApiResponse
     */
     @Suppress("UNCHECKED_CAST")
-    fun uploadFile(petId: kotlin.Long, additionalMetadata: kotlin.String, file: java.io.File) : ApiResponse {
+    fun uploadFile(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : ApiResponse {
         val localVariableBody: kotlin.Any? = mapOf("additionalMetadata" to "$additionalMetadata", "file" to "$file")
         val localVariableQuery: MultiValueMap = mapOf()
         val localVariableHeaders: kotlin.collections.Map<kotlin.String,kotlin.String> = mapOf("Content-Type" to "multipart/form-data")