MPC mo port error

I have 3 measured outputs going into the mo port via a Mux. Yet, I keep getting this error. It's driving me nuts. Could somebody advise?

Error in port widths or dimensions. The signal connected to the "mo" port of the "/MPC Controller" block must be a row or column vector signal with 3 elements.

[Arkadiy Turevskiy]

Are your measured outputs all scalars, can you share your model?