Filter Manager Assert

I get the following assert on a FSCTRL for Compression pre op callback:

*** Assertion failed: FltMgr!FltpHandlePreCallbackReturnStatus:
Pre-operation callbacks can return FLT_PREOP_SYNCHRONIZE only for IRP
operations: FLTP_IS_IRPCTRL_IRP_OPERATION(IrpCtrl)

*** Source File: f:\ntdnsrv\base\fs\filtermgr\filter\callbacksup.c, line
1790

My return statement is as follows:

return (FLT_IS_IRP_OPERATION(Data) ? FLT_PREOP_SYNCHRONIZE
: FLT_PREOP_SUCCESS_WITH_CALLBACK);

Yet I get the assert 8 or more times after the callback returns. The
callback routine is only called once.

Is there any way to tell which pre op callback was actually called?

Thanks,

Ken