diff --git a/lib/utilities/paynym_is_api.dart b/lib/utilities/paynym_is_api.dart
index 629a88f7e..860848cac 100644
--- a/lib/utilities/paynym_is_api.dart
+++ b/lib/utilities/paynym_is_api.dart
@@ -45,7 +45,8 @@ class PaynymIsApi {
 
     final headers = {
       'Content-Type': 'application/json; charset=UTF-8',
-      'Content-Length': contentLength.toString(), // Set the Content-Length header.
+      'content-length':
+          contentLength.toString(), // Set the Content-Length header.
     }..addAll(additionalHeaders);
     final response = await client.post(
       url: uri,