diff --git a/MdePkg/Include/Library/HiiLib.h b/MdePkg/Include/Library/HiiLib.h
index ac2248e810..9bad5b4776 100644
--- a/MdePkg/Include/Library/HiiLib.h
+++ b/MdePkg/Include/Library/HiiLib.h
@@ -155,8 +155,8 @@ HiiLibSetString (
   This function try to retrieve string from String package of current language.
   If fails, it try to retrieve string from String package of first language it support.
 
-  If String is NULL, then ASSERT.
   If StringSize is NULL, then ASSERT.
+  If String is NULL and *StringSize is not 0, then ASSERT.
   If PackageList could not be found in the default HII database, then ASSERT.
   If StringId is not found in PackageList, then ASSERT.