Merge Block Error Simulink

I'm using the merge block, with three input frame based signals, of different frame widths. In order to do this, I checked the box named "allow unequal port widths". However, I get the following error:

Cannot set port 1 of 'xxx/xxx/Merge' to have one dimension when this port is set to be frame-based. For a frame-based port, the number of dimensions must be 2

Any help? Thanks a lot, Luca