diff --git a/lib/matplotlib/font_manager.py b/lib/matplotlib/font_manager.py
index 02cbd9633fcd..af32e8484535 100644
--- a/lib/matplotlib/font_manager.py
+++ b/lib/matplotlib/font_manager.py
@@ -909,7 +909,7 @@ def json_dump(data, filename):
         try:
             json.dump(data, fh, cls=JSONEncoder, indent=2)
         except OSError as e:
-            warnings.warn('Could not save font_manager cache ', e)
+            warnings.warn('Could not save font_manager cache {}'.format(e))
 
 
 def json_load(filename):