diff --git a/src/acpi/acpigen.c b/src/acpi/acpigen.c
index fb92b89f03..e61954fd7f 100644
--- a/src/acpi/acpigen.c
+++ b/src/acpi/acpigen.c
@@ -187,7 +187,7 @@ void acpigen_write_name_unicode(const char *name, const char *string)
 	acpigen_write_len_f();
 	acpigen_write_integer(len);
 	for (size_t i = 0; i < len; i++) {
-		const char c = string[i];
+		const signed char c = string[i];
 		/* Simple ASCII to UTF-16 conversion, replace non ASCII characters */
 		acpigen_emit_word(c >= 0 ? c : '?');
 	}