NW RFC SDK: RFC_INVALID_PARAMETER from RfcGetFunctionDesc

I'm trying to use function RfcGetFunctionDesc from NW RFC SDK for two custom, RFC-enabled functions in SAP. In one case it works all right. But when I try for the second one I get error 20 (RFC_INVALID_PARAMETER). The only thing I change in code is the function name.

What might be the reason why RfcGetFunctionDesc returns error 20 (RFC_INVALID_PARAMETER)? Are the limitations on types of export/import/table parameters or exceptions of function so that it could be inspected by RfcGetFunctionDesc