diff --git a/lib/libc/string/swab.3 b/lib/libc/string/swab.3
index f8ef34ee3f3b..0c9deea8463b 100644
--- a/lib/libc/string/swab.3
+++ b/lib/libc/string/swab.3
@@ -28,7 +28,7 @@
 .\"     @(#)swab.3	8.1 (Berkeley) 6/4/93
 .\" $FreeBSD$
 .\"
-.Dd December 10, 2004
+.Dd March 4, 2012
 .Dt SWAB 3
 .Os
 .Sh NAME
@@ -54,6 +54,9 @@ swapping adjacent bytes.
 The argument
 .Fa len
 must be an even number.
+If
+.Fa len
+is less than zero, nothing will be done.
 .Sh SEE ALSO
 .Xr bzero 3 ,
 .Xr memset 3
diff --git a/lib/libc/string/swab.c b/lib/libc/string/swab.c
index 1db7d1fab00b..ec6cbf0f536f 100644
--- a/lib/libc/string/swab.c
+++ b/lib/libc/string/swab.c
@@ -45,6 +45,8 @@ swab(const void * __restrict from, void * __restrict to, ssize_t len)
 	int n;
 	char *fp, *tp;
 
+	if (len <= 0)
+		return;
 	n = len >> 1;
 	fp = (char *)from;
 	tp = (char *)to;