diff --git a/OpenSSL/_util.py b/OpenSSL/_util.py
index 59a7880..334c850 100644
--- a/OpenSSL/_util.py
+++ b/OpenSSL/_util.py
@@ -55,7 +55,7 @@
         return s
 
 _TEXT_WARNING = (
-    text_type.__name__ + u" for {} is no longer accepted, use bytes"
+    text_type.__name__ + u" for {0} is no longer accepted, use bytes"
 )
 
 def warn_text(label, obj):