diff --git a/modules/openapi-generator/src/main/resources/Java/libraries/resttemplate/ApiClient.mustache b/modules/openapi-generator/src/main/resources/Java/libraries/resttemplate/ApiClient.mustache
index 56ce0754e82..b5de6ce3a19 100644
--- a/modules/openapi-generator/src/main/resources/Java/libraries/resttemplate/ApiClient.mustache
+++ b/modules/openapi-generator/src/main/resources/Java/libraries/resttemplate/ApiClient.mustache
@@ -832,6 +832,9 @@ public class ApiClient{{#jsr310}} extends JavaTimeFormatter{{/jsr310}} {
         }
 
         private String headersToString(HttpHeaders headers) {
+            if(headers == null || headers.isEmpty()) {
+                return "";
+            }
             StringBuilder builder = new StringBuilder();
             for (Entry<String, List<String>> entry : headers.entrySet()) {
                 builder.append(entry.getKey()).append("=[");