diff --git a/checks.c b/checks.c
index c49ed51..2fb7ee5 100644
--- a/checks.c
+++ b/checks.c
@@ -1182,7 +1182,7 @@ static void check_unit_address_format(struct check *c, struct dt_info *dti,
 		/* skip over 0x for next test */
 		unitname += 2;
 	}
-	if (unitname[0] == '0' && isxdigit(unitname[1]))
+	if (unitname[0] == '0' && isxdigit((unsigned char)unitname[1]))
 		FAIL(c, dti, node, "unit name should not have leading 0s");
 }
 WARNING(unit_address_format, check_unit_address_format, NULL,
diff --git a/yamltree.c b/yamltree.c
index 0cb16ed..25fd25e 100644
--- a/yamltree.c
+++ b/yamltree.c
@@ -102,7 +102,7 @@ static void yaml_propval_string(yaml_emitter_t *emitter, char *str, int len)
 
 	/* Make sure the entire string is in the lower 7-bit ascii range */
 	for (i = 0; i < len; i++)
-		assert(isascii(str[i]));
+		assert(isascii((unsigned char)str[i]));
 
 	yaml_scalar_event_initialize(&event, NULL,
 		(const yaml_char_t *)YAML_STR_TAG, (const yaml_char_t*)str,