diff --git a/gnu/usr.bin/man/man/man.c b/gnu/usr.bin/man/man/man.c
index 7c9481cb10b..8edb48c86db 100644
--- a/gnu/usr.bin/man/man/man.c
+++ b/gnu/usr.bin/man/man/man.c
@@ -467,8 +467,13 @@ man_getopt (argc, argv)
 	    || strlen(tmp + 1) < 4
 	    || tmp[3] != '.') {
 		if (debug) {
-			errno = EINVAL;
-			perror ("ctype locale env");
+			if (strcmp(locale, "C") != 0 &&
+			    strcmp(locale, "POSIX") != 0 &&
+			    strcmp(locale, "ASCII") != 0 &&
+			    strcmp(locale, "US-ASCII") != 0) {
+				errno = EINVAL;
+				perror ("ctype locale env");
+			}
 		}
 		locale = NULL;
 	} else {