Error in mupadmex eval function

So I'm trying to do circuit evaluation. Sometimes it evaluates fine and I get solved variables, but often I get the following error:

Error using mupadmex
Error in MuPAD command: Cannot define a matrix over 'Dom::ExpressionField()'.
[(Dom::Matrix(Dom::ExpressionField()))::new]

Error in sym/subsref (line 687)
        B = mupadmex('symobj::subsref',A.s,inds{:});

Error in CircuitGA (line 327)
                eval([char(X(i)) '=' char(V(i)) ';']);

Does anyone know why this is happening? Is there a fix?

[Christopher Creutzig]

It's unlikely anyone can know that without you providing a minimal complete self-contained example that shows the bug.

From the snippet you sent, I assume the primary problem is the use of char and eval. That is fragile and should not be necessary for symbolic computations in the first place.