COMSOL external MATLAB function

Hello forum.

I want to use an external matlab function to define a local heat transfer coefficient distribution.

If I use a very simple external function, everything works fine:

for example:

function value=alpha(z)
value = ones(size(z))*65000;
end

My function will load an object with the current data from a *mat - file and will read the data and give it back to COMSOL
something like that:

function out=AlphaZ1(z)

global pipe1;
path('V:\30_Numerik\COMSOL\Thermosyphon_2.0alpha\20130218001\classes',path)
strStoredVariableDataFileName = 'V:\30_Numerik\COMSOL\Thermosyphon_2.0alpha\20130218001\pipe1.mat';
load(strStoredVariableDataFileName)
value=pipe1.CondenserSection.GetAlphaZ(z');
out=round(value');

end

Unfortunately I always get an error message:

Attempted to access value(:,1); index out of bounds because size(value)=[0,0].
with no further details

If I save the z - vector to an mat-file and later try to call the function with this z-vector, it works properly and I get the correct heat transfer distribution.

Is there a way to analyse the error message and to find where it occurs? I already tried to catch all errors with a try-catch statement in MATLAB, but that does not help

Matthias