diff --git a/drivers/regulator/regulator_shell.c b/drivers/regulator/regulator_shell.c
index 2f17a712521..2e453b9701d 100644
--- a/drivers/regulator/regulator_shell.c
+++ b/drivers/regulator/regulator_shell.c
@@ -35,7 +35,7 @@ static int strtomicro(char *inp, char units, int32_t *val)
 	} else if ((len > 2) && (inp[len - 2] == 'm')) {
 		mult = 1000;
 		end = len - 3;
-	} else if (isdigit(inp[len - 2]) > 0) {
+	} else if (isdigit((unsigned char)inp[len - 2]) > 0) {
 		mult = 1000000;
 		end = len - 2;
 	} else {
@@ -55,7 +55,7 @@ static int strtomicro(char *inp, char units, int32_t *val)
 	/* numeric part */
 	*val = 0;
 	for (size_t i = start; i <= end; i++) {
-		if (isdigit(inp[i]) > 0) {
+		if (isdigit((unsigned char)inp[i]) > 0) {
 			*val = *val * 10 / decdiv +
 			       (int32_t)(inp[i] - '0') * mult / decdiv;
 		} else if (inp[i] == '.') {