About interlaced encoding...

Hello!
When i set NV_ENC_CONFIG::frameFieldMode to NV_ENC_PARAMS_FRAME_FIELD_MODE_FIELD and pass NV_ENC_PIC_STRUCT_FIELD_TOP_BOTTOM to NV_ENC_PIC_PARAMS, the encoder never produce i-frame or idr-frame, all frames are p-frame and b-frame (pictureType of NV_ENC_LOCK_BITSTREAM). why?