Multiplexer with variable size signals

I am using MATLAB/simulink 2014a and I would like to feed a multiplexer (with 3 ports) with 3 signals which have variable-sizes. But I get the error:

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Someone could help me to solve the problem?

Thanks a lot.

Solution

First of all thanks for the comments. They helped. I try to write a comprehensive answer in this way if someone has my same question, he/she will find here a possible answer.

MULTIPLEXER in SIMULINK don't support Variable-Size signals. You can give a look to which blocks of Simulink support variable-size signals here: http://it.mathworks.com/help/simulink/ug/simulink-block-support-for-variable-size-signals.html I think the reason is that the components of a mux signal must all have the same data type and cannot be hierarchical.
To workaround this problem one way would be to use the block BUS CREATOR, the following: http://it.mathworks.com/help/simulink/slref/buscreator.html
To be more clear you can give a look at the differences between MUX(demux) and bus creator here: http://fr.mathworks.com/matlabcentral/answers/97787-what-is-the-difference-between-the-mux-demux-and-bus-creator-bus-selector-blocks-in-simulink

I hope this will help someone.