diff --git a/src/utils.c b/src/utils.c
index 7b7178c..7364bfb 100644
--- a/src/utils.c
+++ b/src/utils.c
@@ -999,7 +999,7 @@
 
 	/* All input must be numbers */
 	for (i = 0; i < in_len; i++) {
-		if (!isdigit(in[i]))
+		if (!isdigit((unsigned char)in[i]))
 			return -EINVAL;
 	}