diff --git a/reactos/lib/rtl/unicode.c b/reactos/lib/rtl/unicode.c
index 04ce389200a..3686415c81d 100644
--- a/reactos/lib/rtl/unicode.c
+++ b/reactos/lib/rtl/unicode.c
@@ -1943,6 +1943,8 @@ RtlCreateUnicodeString(
     PAGED_CODE_RTL();
 
     Length = (wcslen(Source) + 1) * sizeof(WCHAR);
+    if (Length > 0xFFFE) return FALSE;
+
     UniDest->Buffer = RtlpAllocateStringMemory(Length, TAG_USTR);
 
     if (UniDest->Buffer == NULL) return FALSE;