It's a bit hard to say as you don't show the code in your question, but it may be that you have not assigned a value to the output argument "y" (and maybe others) in your function thestest.
That is, if your function starts
function y = thestest(...)
there should be a line in it somewhere that looks like
y = ... ;
You should perhaps also look at the structure of the m-file. Does it contain more than one function definition?
