diff --git a/dlls/msvcrtd/debug.c b/dlls/msvcrtd/debug.c
index 04720588374..7e2fa0daf0f 100644
--- a/dlls/msvcrtd/debug.c
+++ b/dlls/msvcrtd/debug.c
@@ -31,16 +31,19 @@ int _crtAssertBusy = -1;
 int _crtBreakAlloc = -1;
 int _crtDbgFlag = 0;
 
-extern int _callnewh(unsigned long);
+#ifdef _WIN64
+typedef unsigned __int64 MSVCRT_size_t;
+#else
+typedef unsigned long MSVCRT_size_t;
+#endif
+
+extern int _callnewh(MSVCRT_size_t);
 
 /*********************************************************************
  *		??2@YAPAXIHPBDH@Z (MSVCRTD.@)
  */
-void * CDECL MSVCRTD_operator_new_dbg(
-	unsigned long nSize,
-	int nBlockUse,
-	const char *szFileName,
-	int nLine)
+void * CDECL MSVCRTD_operator_new_dbg(MSVCRT_size_t nSize, int nBlockUse,
+                                      const char *szFileName, int nLine)
 {
     void *retval = NULL;
 
diff --git a/dlls/msvcrtd/tests/debug.c b/dlls/msvcrtd/tests/debug.c
index fbfcdc5ea4d..a7f7db1d82b 100644
--- a/dlls/msvcrtd/tests/debug.c
+++ b/dlls/msvcrtd/tests/debug.c
@@ -30,7 +30,7 @@
 
 /**********************************************************************/
 
-static void * (*pMSVCRTD_operator_new_dbg)(unsigned long, int, const char *, int) = NULL;
+static void * (*pMSVCRTD_operator_new_dbg)(size_t, int, const char *, int) = NULL;
 
 /* Some exports are only available in later versions */
 #define SETNOFAIL(x,y) x = (void*)GetProcAddress(hModule,y)