diff --git a/api/onnx_web/output.py b/api/onnx_web/output.py
index ddcb0e29..4b3291af 100644
--- a/api/onnx_web/output.py
+++ b/api/onnx_web/output.py
@@ -37,7 +37,7 @@ def json_params(
         json['border'] = border.tojson()
         size = size.add_border(border)
 
-    json['size'] = size
+    json['size'] = size.tojson()
 
     return json