diff --git a/include/efi_selftest.h b/include/efi_selftest.h
index e900cb85a9..7c69c3f376 100644
--- a/include/efi_selftest.h
+++ b/include/efi_selftest.h
@@ -131,7 +131,7 @@ u16 *efi_st_translate_code(u16 code);
  * @buf2:	char string
  * Return:	0 if both buffers contain equivalent strings
  */
-int efi_st_strcmp_16_8(const u16 *buf1, const char *buf2);
+int efi_st_strcmp_16_8(const u16 *buf1, const unsigned char *buf2);
 
 /**
  * efi_st_get_config_table() - get configuration table
diff --git a/lib/efi_selftest/efi_selftest_util.c b/lib/efi_selftest/efi_selftest_util.c
index 7e03e0c939..3681fa6431 100644
--- a/lib/efi_selftest/efi_selftest_util.c
+++ b/lib/efi_selftest/efi_selftest_util.c
@@ -102,7 +102,7 @@ u16 *efi_st_translate_code(u16 code)
 	return efi_st_unknown;
 }
 
-int efi_st_strcmp_16_8(const u16 *buf1, const char *buf2)
+int efi_st_strcmp_16_8(const u16 *buf1, const unsigned char *buf2)
 {
 	for (; *buf1 || *buf2; ++buf1, ++buf2) {
 		if (*buf1 != *buf2)