Change variable-size mode to fixed-size mode in Simulink

Hi

i work with images/videos. so i detect in a video an object with the "blob analyse" block and i will use the centroid. so far so good. now i will track the object with an kalman filter. so i want to put the centroid coordinates to the "kalman filter" block.

it looks like this block can't handle variable-size mode signals.

but the blob analyse can only provide variable-size mode signals, this is for me clear because sometimes i don't recognize the object. iT's clear that i should handle this.

so my problem is now how i can change the variable-size mode to fixed-size mode. in an blog or tutorial i read that the "Assignment" block could do that, but i only receive an error "The dimensions mode of input port U in '*/Assignment' does not match the expected dimensions mode based on the dialog parameters and the dimensions mode of other ports"

does someone know a solution for that ?