compare(data,sys) can not be used inside function?

Hallo @ all.

I think I stumbled upon a bug in Matlab R2011b:

My goal was to compare the fit of different estimated models to measured data by using the function compare(data, sys) from ident toolbox and something strange happened. Although I am able to use the function in 'base' Workspace

[~, fit, ~] = compare(dataPack{1,1}, b)
fit =
   90.5454

I can not use it inside another user defined function

function fit = fitOfModel(iddataObj, varargin )
[~, fit, ~] = compare(iddataObj,varargin);
end

calling this function with the same input as before returns an error

[~, fit, ~] = fitOfModel(dataPack{1,1}, b)
Undefined function 'compare' for input arguments of type 'iddata'.

I have no idea as to why this might happen. Could someone please enlighten me? Is this a bug?

Ps.: dataPack is a cell array where each cell contains an iddata object with data from experiments and b is the estimated P1D idproc model for the data in dataPack{1,1}