Mocking a Set Method doesn't work

I want to Mock a set method while conducting unit tests. when I try to mock it, I get the error:

Unrecognized method, property, or field 'set' for class

Any help is appreciated.

The mock is defined as follows:

testCase.assignOutputsWhen(...
        withAnyInputs(behaviorObj.set.blablabla),DoNothing);

The Set method is defined as follows:

function set.blablabla(obj)
    arguments
    end
    %"stuf"
end