diff --git a/src/str.c b/src/str.c
index ea0aa3c..24a0900 100644
--- a/src/str.c
+++ b/src/str.c
@@ -66,7 +66,7 @@ pass:	for(argv += 2, r = 1; *argv != NULL; ++argv)
 			/* First checks if argv[0][i] is valid ASCII; ctypes(3)
 			 * don't handle non-ASCII.
 			 * This is bad. */
-			if(argv[0][i] < 0x80 && !ctypes[ctype].f(argv[0][i]))
+			if((unsigned char)argv[0][i] < 0x80 && !ctypes[ctype].f(argv[0][i]))
 				return 1;
 			else
 				r = 0;