diff --git a/src/include/readline/readline.h b/src/include/readline/readline.h
index 4214a44f..79123130 100644
--- a/src/include/readline/readline.h
+++ b/src/include/readline/readline.h
@@ -7,6 +7,6 @@
  *
  */
 
-extern char * readline ( const char *prompt );
+extern char * readline ( const char *prompt ) __nonnull;
 
 #endif /* _READLINE_H */