FMOD_ERR_INVALID_PARAM when adding DSP to ChannelGroup

I keep getting FMOD_ERR_INVALID_PARAM when trying to add DSP of type FMOD_DSP_TYPE_LOWPASS to custom ChannelGroup.
I double checked that the DSP is created successfully and ChannelGroup is initialized properly. What else could cause this error?

FMOD::ChannelGroup master = 0;

FMOD::ChannelGroup worldSfxGroup = 0;
CHECK_ERR(system->createChannelGroup("World", &worldSfxGroup));

FMOD::DSP lpf;
CHECK_ERR(worldSfxGroup->addDSP(2, lpf)); //FMOD_ERR_INVALID_PARAM on this line

The first parameter of ChannelGroup::addDSP() is a FMOD_CHANNELCONTROL_DSP_INDEX and ‘2’ is not a valid value.