Index: /trunk/src/VBox/Runtime/r3/fileio.cpp
===================================================================
--- /trunk/src/VBox/Runtime/r3/fileio.cpp	(revision 58824)
+++ /trunk/src/VBox/Runtime/r3/fileio.cpp	(revision 58825)
@@ -144,5 +144,9 @@
      * Validate                                                                                                                                       .
      */
+#ifdef RT_OS_WINDOWS
+    AssertMsgReturn((fOpen & RTFILE_O_ACCESS_MASK) || (fOpen & RTFILE_O_ACCESS_MASK) == RTFILE_O_ATTR_ONLY, ("Missing RTFILE_O_READ/WRITE/ATTR_ONLY: fOpen=%#llx\n", fOpen), VERR_INVALID_PARAMETER);
+#else
     AssertMsgReturn(fOpen & RTFILE_O_ACCESS_MASK, ("Missing RTFILE_O_READ/WRITE: fOpen=%#llx\n", fOpen), VERR_INVALID_PARAMETER);
+#endif
 #if defined(RT_OS_WINDOWS) || defined(RT_OS_OS2)
     AssertMsgReturn(!(fOpen & (~(uint64_t)RTFILE_O_VALID_MASK | RTFILE_O_NON_BLOCK)), ("%#llx\n", fOpen), VERR_INVALID_PARAMETER);