diff --git a/modules/openapi-generator/src/main/resources/kotlin-client/libraries/jvm-spring-restclient/infrastructure/ApiClient.kt.mustache b/modules/openapi-generator/src/main/resources/kotlin-client/libraries/jvm-spring-restclient/infrastructure/ApiClient.kt.mustache
index 62876703d700..ec9ac2f6f2ef 100644
--- a/modules/openapi-generator/src/main/resources/kotlin-client/libraries/jvm-spring-restclient/infrastructure/ApiClient.kt.mustache
+++ b/modules/openapi-generator/src/main/resources/kotlin-client/libraries/jvm-spring-restclient/infrastructure/ApiClient.kt.mustache
@@ -46,8 +46,23 @@ import org.springframework.util.LinkedMultiValueMap
     private fun <I> RestClient.RequestBodySpec.headers(requestConfig: RequestConfig<I>) =
         apply { requestConfig.headers.forEach { (name, value) -> header(name, value) } }
 
-    private fun <I : Any> RestClient.RequestBodySpec.nullableBody(requestConfig: RequestConfig<I>) =
-        apply { if (requestConfig.body != null) body(requestConfig.body) }
+    private fun <I : Any> RestClient.RequestBodySpec.nullableBody(requestConfig: RequestConfig<I>): RestClient.RequestBodySpec {
+        when {
+            requestConfig.headers[HttpHeaders.CONTENT_TYPE] == MediaType.MULTIPART_FORM_DATA_VALUE -> {
+                val parts = LinkedMultiValueMap<String, Any>()
+                (requestConfig.body as Map<String, PartConfig<*>>).forEach { (name, part) ->
+                    if (part.body != null) {
+                        parts.add(name, part.body)
+                    }
+                }
+                return apply { body(parts) }
+            }
+
+            else -> {
+                return apply { if (requestConfig.body != null) body(requestConfig.body) }
+            }
+        }
+    }
 }
 
 {{^nonPublicApi}}{{#explicitApi}}public {{/explicitApi}}{{/nonPublicApi}}inline fun <reified T: Any> parseDateToQueryString(value : T): String {
diff --git a/samples/client/echo_api/kotlin-jvm-spring-3-restclient/src/main/kotlin/org/openapitools/client/infrastructure/ApiClient.kt b/samples/client/echo_api/kotlin-jvm-spring-3-restclient/src/main/kotlin/org/openapitools/client/infrastructure/ApiClient.kt
index 64f843ed93be..426ad04918f9 100644
--- a/samples/client/echo_api/kotlin-jvm-spring-3-restclient/src/main/kotlin/org/openapitools/client/infrastructure/ApiClient.kt
+++ b/samples/client/echo_api/kotlin-jvm-spring-3-restclient/src/main/kotlin/org/openapitools/client/infrastructure/ApiClient.kt
@@ -46,8 +46,23 @@ open class ApiClient(protected val client: RestClient) {
     private fun <I> RestClient.RequestBodySpec.headers(requestConfig: RequestConfig<I>) =
         apply { requestConfig.headers.forEach { (name, value) -> header(name, value) } }
 
-    private fun <I : Any> RestClient.RequestBodySpec.nullableBody(requestConfig: RequestConfig<I>) =
-        apply { if (requestConfig.body != null) body(requestConfig.body) }
+    private fun <I : Any> RestClient.RequestBodySpec.nullableBody(requestConfig: RequestConfig<I>): RestClient.RequestBodySpec {
+        when {
+            requestConfig.headers[HttpHeaders.CONTENT_TYPE] == MediaType.MULTIPART_FORM_DATA_VALUE -> {
+                val parts = LinkedMultiValueMap<String, Any>()
+                (requestConfig.body as Map<String, PartConfig<*>>).forEach { (name, part) ->
+                    if (part.body != null) {
+                        parts.add(name, part.body)
+                    }
+                }
+                return apply { body(parts) }
+            }
+
+            else -> {
+                return apply { if (requestConfig.body != null) body(requestConfig.body) }
+            }
+        }
+    }
 }
 
 inline fun <reified T: Any> parseDateToQueryString(value : T): String {
diff --git a/samples/client/petstore/kotlin-jvm-spring-3-restclient/src/main/kotlin/org/openapitools/client/infrastructure/ApiClient.kt b/samples/client/petstore/kotlin-jvm-spring-3-restclient/src/main/kotlin/org/openapitools/client/infrastructure/ApiClient.kt
index 64f843ed93be..426ad04918f9 100644
--- a/samples/client/petstore/kotlin-jvm-spring-3-restclient/src/main/kotlin/org/openapitools/client/infrastructure/ApiClient.kt
+++ b/samples/client/petstore/kotlin-jvm-spring-3-restclient/src/main/kotlin/org/openapitools/client/infrastructure/ApiClient.kt
@@ -46,8 +46,23 @@ open class ApiClient(protected val client: RestClient) {
     private fun <I> RestClient.RequestBodySpec.headers(requestConfig: RequestConfig<I>) =
         apply { requestConfig.headers.forEach { (name, value) -> header(name, value) } }
 
-    private fun <I : Any> RestClient.RequestBodySpec.nullableBody(requestConfig: RequestConfig<I>) =
-        apply { if (requestConfig.body != null) body(requestConfig.body) }
+    private fun <I : Any> RestClient.RequestBodySpec.nullableBody(requestConfig: RequestConfig<I>): RestClient.RequestBodySpec {
+        when {
+            requestConfig.headers[HttpHeaders.CONTENT_TYPE] == MediaType.MULTIPART_FORM_DATA_VALUE -> {
+                val parts = LinkedMultiValueMap<String, Any>()
+                (requestConfig.body as Map<String, PartConfig<*>>).forEach { (name, part) ->
+                    if (part.body != null) {
+                        parts.add(name, part.body)
+                    }
+                }
+                return apply { body(parts) }
+            }
+
+            else -> {
+                return apply { if (requestConfig.body != null) body(requestConfig.body) }
+            }
+        }
+    }
 }
 
 inline fun <reified T: Any> parseDateToQueryString(value : T): String {