diff --git a/procrunner/__init__.py b/procrunner/__init__.py
index 9f827d4..6d9ab6d 100644
--- a/procrunner/__init__.py
+++ b/procrunner/__init__.py
@@ -336,7 +336,7 @@
         if key in self._extras:
             return self._extras[key]
         if not hasattr(self, key):
-            raise KeyError(f"Unknown attribute {key}")
+            raise KeyError("Unknown attribute {key}".format(key=key))
         return getattr(self, key)
 
     def __eq__(self, other):