Error due to variable size data in Matlab function block

I have a problem that many others had before, so I read some discussions and references before asking the following: I have a Matlab function block in Simulink, which would like to be a modulator. It gets as input a [nx1] frame of data and should return a an [lx1] frame, where either l=n or l = n/K, for some K which divides n. However, the (very simplyfied extract of) code   function ak = Modulator(dataFrame,dataType) coder.varsize('ak',length(dataFrame)); M = 4; if dataType == 1 %input is a binary stream, bit mapping required ak = zeros(round(length(dataFrame)/log2(M)),1); else % input data is a stream of integer ak = zeros(length(dataFrame),1); end end "Data 'ak' is inferred as a variable size matrix, while its specified type is something else." Now, in line 2 I specified that it is a variable size matrix, and I also used an if/else constructor to initialize it. To make the compiler happy, one may check the "Variable number of columns" checkbox for ak, in the Data and Ports Manager, but this turns out in a new error, because the blocks in cascade don't accept variable data, at least the ones I need, like digital filters.