diff --git a/base/services/umpnpmgr/umpnpmgr.c b/base/services/umpnpmgr/umpnpmgr.c
index 2352efe703f..b06f1c45c8b 100644
--- a/base/services/umpnpmgr/umpnpmgr.c
+++ b/base/services/umpnpmgr/umpnpmgr.c
@@ -137,7 +137,7 @@ RpcServerThread(LPVOID lpParameter)
 }
 
 
-void __RPC_FAR * __RPC_USER midl_user_allocate(SIZE_T len)
+void __RPC_FAR * __RPC_USER midl_user_allocate(size_t len)
 {
     return HeapAlloc(GetProcessHeap(), HEAP_ZERO_MEMORY, len);
 }