RFC Error While Invoking A Remote Function Module.

I am invoking A remote function module while invoking it get an exception when i see the error logs

com.sap.tc.webdynpro.modelimpl.dynamicrfc.WDDynamicRFCExecuteException: Screen output without connection to user. , error key: RFC_ERROR_SYSTEM_FAILURE at com.sap.tc.webdynpro.modelimpl.dynamicrfc.DynamicRFCModelClassExecutable.execute(DynamicRFCModelClassExecutable.java:101) at... [see details]

can anyone pls tell me what the error is..