diff --git a/reactos/sdk/lib/rtl/unicode.c b/reactos/sdk/lib/rtl/unicode.c
index 2441a1f9c36..99357e272a3 100644
--- a/reactos/sdk/lib/rtl/unicode.c
+++ b/reactos/sdk/lib/rtl/unicode.c
@@ -515,7 +515,6 @@ RtlIsValidOemCharacter(IN PWCHAR Char)
 {
     WCHAR UnicodeChar;
     WCHAR OemChar;
-    UCHAR Index;
 
     /* If multi-byte code page present */
     if (NlsMbOemCodePageTag)
@@ -528,19 +527,21 @@ RtlIsValidOemCharacter(IN PWCHAR Char)
         if (NlsOemLeadByteInfo[HIBYTE(OemChar)])
             Offset = NlsOemLeadByteInfo[HIBYTE(OemChar)];
 
-        Index = LOBYTE(OemChar) + Offset;
+        /* Receive Unicode character from the table */
+        UnicodeChar = RtlUpcaseUnicodeChar(NlsOemToUnicodeTable[LOBYTE(OemChar) + Offset]);
+
+        /* Receive OEM character from the table */
+        OemChar = NlsUnicodeToMbOemTable[UnicodeChar];
     }
     else
     {
-        Index = NlsUnicodeToOemTable[*Char];
+        /* Receive Unicode character from the table */
+        UnicodeChar = RtlUpcaseUnicodeChar(NlsOemToUnicodeTable[(UCHAR)NlsUnicodeToOemTable[*Char]]);
+
+        /* Receive OEM character from the table */
+        OemChar = NlsUnicodeToOemTable[UnicodeChar];
     }
 
-    /* Receive Unicode character from the table */
-    UnicodeChar = RtlUpcaseUnicodeChar(NlsOemToUnicodeTable[Index]);
-
-    /* Receive OEM character from the table */
-    OemChar = NlsUnicodeToOemTable[UnicodeChar];
-
     /* Not valid character, failed */
     if (OemChar == NlsOemDefaultChar)
         return FALSE;