diff --git a/MokManager.c b/MokManager.c
index cd569b6..1bf2333 100644
--- a/MokManager.c
+++ b/MokManager.c
@@ -1913,8 +1913,7 @@ static BOOLEAN check_der_suffix(CHAR16 * file_name)
 	if (!file_name || StrLen(file_name) <= 4)
 		return FALSE;
 
-	suffix[0] = '\0';
-	StrnCat(suffix, file_name + StrLen(file_name) - 4, 4);
+	StrCpy(suffix, file_name + StrLen(file_name) - 4);
 
 	StrLwr(suffix);
 	for (i = 0; der_suffix[i] != NULL; i++) {