msk demodulation

Iam trying to do MSK demodulation on GSM signals like

checkWarn (ModtMSKDemodulate (
                mtSession,  resampledT0,resampledDt,  resampledWavefrom, numResampledWaveform,  enableDifferentialEncoding, samplesPerSymbol,  NULL, matchedFilterCoefficents,
                numMatchedFilterCoefficients, PulseShapingFilterCoefficients,  numPulseShapingFilterCoefficients,  reset, &mskRecoveredDataLength, &mskDemodulatedDataLength,&mskMeasurements));

 Then trying to recover the demodulated bit stream with this function

 if(mskDemodulatedDataLength > oldNumRecoverdBitStream)
	{
		free(recoverdBitStream);
		recoverdBitStream = malloc(mskDemodulatedDataLength * sizeof(unsigned char));
		oldNumRecoverdBitStream = mskDemodulatedDataLength;
	}    
    checkWarn (ModtGetRecoveredBitStream ( mtSession,mskDemodulatedDataLength,recoverdBitStream));

 This returns an error message of " Null pointer argument to library function" and surely recoverdBitStream is been returned as NULL. But i try to check where the error is i cant find it since its sister function

	if(mskRecoveredDataLength > oldNumRecoveredWaveform)
	{
		free(recoverdWaveform);	
		recoverdWaveform = malloc(mskRecoveredDataLength * sizeof(niComplexNum));
	oldNumRecoveredWaveform = mskRecoveredDataLength;
	}
    checkWarn (ModtGetRecoveredComplexWaveform (
         mtSession,&recoverdWaveformT0,&recoverdWaveformDt,mskRecoveredDataLength,recoverdWaveform));

 is giving the likely results.