diff --git a/doc/sh-utils.texi b/doc/sh-utils.texi
index 442d9b617..14b445bd2 100644
--- a/doc/sh-utils.texi
+++ b/doc/sh-utils.texi
@@ -840,7 +840,7 @@ An alternative way to do pattern matching.  This is the same as
 @findex substr
 Returns the substring of @var{string} beginning at @var{position}
 with length at most @var{length}.  If either @var{position} or
-@var{length} is negative or non-numeric, returns the null string.
+@var{length} is negative, zero, or non-numeric, returns the null string.
 
 @item index @var{string} @var{character-class}
 @findex index