Package function not allowed in fittype?

I have a function called 'linepowerthreshold' that I use to fit a function. I recently made it into a package and put it in the directory:

+vlt/+fit/linepowerthreshold.m

While the following works great, when linepowerthreshold.m was just an m file not inside a package:

ft = fittype('linepowerthreshold(x,a,0,b,1);','options',s);

The following contruction fails in Matlab 2020a (MacOS, Mojave):

ft = fittype('vlt.fit.linepowerthreshold(x,a,0,b,1);','options',s);

Error using fittype/testCustomModelEvaluation (line 12)
    Expression vlt.fit.linepowerthreshold(x,a,0,b,1); is not a valid MATLAB expression, has non-scalar coefficients, or cannot be evaluated:
    Error in fittype expression ==> vlt.fit.linepowerthreshold(x,a,0,b,1);
    ??? Dot indexing is not supported for variables of this type.

This is a workaround but it seems like passing a package-based function should work, too, yes?

    ft = fittype(@(a,b,x) vlt.fit.linepowerthreshold(x,a,0,b,1),'options',s);

The function when called alone works fine, and is byte-by-byte the same as the version that is not in a package (except for the documentation).

Help?