diff --git a/rtl/objpas/sysutils/disk.inc b/rtl/objpas/sysutils/disk.inc
index 7f80d64f1d..277156ace7 100644
--- a/rtl/objpas/sysutils/disk.inc
+++ b/rtl/objpas/sysutils/disk.inc
@@ -19,7 +19,7 @@ begin
   if length(ADrive)=0 then
     Result:=0
   else
-    Result := (Ord(ADrive[1]))-64;
+    Result := Ord(UpCase(ADrive[1]))-64;
 end;
 
 {$push}