diff --git a/MdePkg/Library/BaseLib/SafeString.c b/MdePkg/Library/BaseLib/SafeString.c
index 29310889ca..417497cbc9 100644
--- a/MdePkg/Library/BaseLib/SafeString.c
+++ b/MdePkg/Library/BaseLib/SafeString.c
@@ -2932,7 +2932,7 @@ AsciiStrToUnicodeStrS (
   // Convert string
   //
   while (*Source != '\0') {
-    *(Destination++) = (CHAR16)*(Source++);
+    *(Destination++) = (CHAR16)(UINT8)*(Source++);
   }
   *Destination = '\0';
 
@@ -3045,7 +3045,7 @@ AsciiStrnToUnicodeStrS (
   // Convert string
   //
   while ((*Source != 0) && (SourceLen > 0)) {
-    *(Destination++) = (CHAR16)*(Source++);
+    *(Destination++) = (CHAR16)(UINT8)*(Source++);
     SourceLen--;
     (*DestinationLength)++;
   }
diff --git a/MdePkg/Library/BaseLib/String.c b/MdePkg/Library/BaseLib/String.c
index cb90774c86..e6df12797d 100644
--- a/MdePkg/Library/BaseLib/String.c
+++ b/MdePkg/Library/BaseLib/String.c
@@ -1746,7 +1746,7 @@ AsciiStrToUnicodeStr (
 
   ReturnValue = Destination;
   while (*Source != '\0') {
-    *(Destination++) = (CHAR16) *(Source++);
+    *(Destination++) = (CHAR16)(UINT8) *(Source++);
   }
   //
   // End the Destination with a NULL.