diff --git a/cmd/acpi.c b/cmd/acpi.c
index c543f1e..0e473b4 100644
--- a/cmd/acpi.c
+++ b/cmd/acpi.c
@@ -178,7 +178,7 @@
 		printf("Table name '%s' must be four characters\n", name);
 		return CMD_RET_FAILURE;
 	}
-	str_to_upper(name, sig, -1);
+	str_to_upper(name, sig, ACPI_NAME_LEN);
 	ret = dump_table_name(sig);
 	if (ret) {
 		printf("Table '%.*s' not found\n", ACPI_NAME_LEN, sig);