diff --git a/reactos/include/ddk/winddk.h b/reactos/include/ddk/winddk.h
index f43603f9952..4bab620ed9f 100644
--- a/reactos/include/ddk/winddk.h
+++ b/reactos/include/ddk/winddk.h
@@ -9277,7 +9277,7 @@ MmSecureVirtualMemory(
   IN ULONG  ProbeMode);
 
 NTKERNELAPI
-ULONG
+SIZE_T
 NTAPI
 MmSizeOfMdl(
   IN PVOID  Base,