diff --git a/MdePkg/Library/BaseLib/String.c b/MdePkg/Library/BaseLib/String.c
index 53ff730e9e..a389115d71 100644
--- a/MdePkg/Library/BaseLib/String.c
+++ b/MdePkg/Library/BaseLib/String.c
@@ -2070,7 +2070,7 @@ Base64Decode (
       Chr = DecodingTable[(UINT8) Source[SourceIndex++]];
       } while (Chr == BAD_V);
       Value <<= 6;
-      Value |= Chr;
+      Value |= (UINT32)Chr;
     }
 
     //