diff --git a/src/editfns.c b/src/editfns.c
index e67e56d2efa..c8b02570c12 100644
--- a/src/editfns.c
+++ b/src/editfns.c
@@ -3424,7 +3424,7 @@ usage: (format STRING &rest OBJECTS)  */)
 	   digits to print after the '.' for floats, or the max.
 	   number of chars to print from a string.  */
 
-	while (index ("-0# ", *format))
+	while (*format && index ("-0# ", *format))
 	  ++format;
 
 	if (*format >= '0' && *format <= '9')