MFX_ERR_NOT_ENOUGH_BUFFER

I am trying to figure out why this command gives a MFX_ERR_NOT_ENOUGH_BUFFER error:

sts = mfxENC->EncodeFrameAsync(NULL, YUVFrame, &mfxBS, &syncp);

YUVFrame has Y,U, and V pointing to buffers and the format is MFX_CHROMAFORMAT_YUV420 and MFX_FOURCC_NV12

mfxBS is the mfxBitstream and seems to have Data initialized and has a MaxLength of 375000

Thank you!