diff --git a/funcs/func_strings.c b/funcs/func_strings.c
index 9af4dfca4f..2bd47c3de1 100644
--- a/funcs/func_strings.c
+++ b/funcs/func_strings.c
@@ -459,10 +459,10 @@ static int filter(struct ast_channel *chan, const char *cmd, char *parse, char *
 
 		if (*(args.allowed) == '-') {
 			if (ast_get_encoded_char(args.allowed + 1, &c2, &consumed))
-				c2 = -1;
+				c2 = c1;
 			args.allowed += consumed + 1;
 
-			if ((c2 < c1 || c2 == -1) && !ast_opt_dont_warn) {
+			if ((unsigned char) c2 < (unsigned char) c1 && !ast_opt_dont_warn) {
 				ast_log(LOG_WARNING, "Range wrapping in FILTER(%s,%s).  This may not be what you want.\n", parse, args.string);
 			}
 
@@ -470,7 +470,7 @@ static int filter(struct ast_channel *chan, const char *cmd, char *parse, char *
 			 * Looks a little strange, until you realize that we can overflow
 			 * the size of a char.
 			 */
-			for (ac = c1; ac != c2; ac++) {
+			for (ac = (unsigned char) c1; ac != (unsigned char) c2; ac++) {
 				bitfield[ac / 32] |= 1 << (ac % 32);
 			}
 			bitfield[ac / 32] |= 1 << (ac % 32);