diff --git a/wlauto/result_processors/ipynb_exporter/__init__.py b/wlauto/result_processors/ipynb_exporter/__init__.py
index 8272a517..12cdc42b 100644
--- a/wlauto/result_processors/ipynb_exporter/__init__.py
+++ b/wlauto/result_processors/ipynb_exporter/__init__.py
@@ -207,7 +207,7 @@ class IPythonNotebookExporter(ResultProcessor):
                 out.evalue = content["evalue"]
                 out.traceback = content["traceback"]
             else:
-                raise ValueError("Unknown msg_type %s".format(msg_type))
+                raise ValueError("Unknown msg_type {}".format(msg_type))
 
             outs.append(out)