diff --git a/lib/libc/string/swab.3 b/lib/libc/string/swab.3
index 13be3ebc4755..7efc22f8bfbb 100644
--- a/lib/libc/string/swab.3
+++ b/lib/libc/string/swab.3
@@ -55,7 +55,7 @@ swapping adjacent bytes.
 .Pp
 The argument
 .Fa len
-must be even number.
+must be an even number.
 .Sh SEE ALSO
 .Xr bzero 3 ,
 .Xr memset 3