diff --git a/programs/winecfg/drive.c b/programs/winecfg/drive.c
index b0e5082d69e..83bcde27a22 100644
--- a/programs/winecfg/drive.c
+++ b/programs/winecfg/drive.c
@@ -73,7 +73,7 @@ long drive_available_mask(char letter)
   for(i = 0; i < 26; i++)
   {
       if (!drives[i].in_use) continue;
-      result |= (1 << (toupper(drives[i].letter) - 'A'));
+      result |= (1 << (letter_to_index(drives[i].letter)));
   }
 
   result = ~result;