diff --git a/source/i3-support.c b/source/i3-support.c
index dc37b2e3..d0d7748b 100644
--- a/source/i3-support.c
+++ b/source/i3-support.c
@@ -57,7 +57,7 @@ void i3_support_focus_window ( Window id )
     char               command[upm];
 
     if ( strlen ( i3_socket_path ) > upm ) {
-        fprintf ( stderr, "Socket path is too long. %zd > %zd\n", strlen ( i3_socket_path ), upm );
+        fprintf ( stderr, "Socket path is too long. %zu > %zu\n", strlen ( i3_socket_path ), upm );
         return;
     }