Undefined function 'eq' for input arguments of type 'struct' using any custom equation in curve fitting tool

Matlab 2016a on Win10.

I'm experiencing a weird problem with the curve fitting tool: whenever I try to fit to a custom equation, the result is "Undefined function 'eq' for input arguments of type 'struct'". It happens even if I do a clean start of Matlab, define two simple vectors (e.g. x = 1:10, y = x.^2) and then try to fit to a custom equation, whatever the equation is.

I suspect that somewhere a symbol as been redefined and breaks the fitting tool code, but my startup.m is empty and I don't know where else to look.

To debug the issue, I tried to do the same thing programatically. To avoid mistakes on my end, I just copied a matlab help example, but got the same error when defining the fittype:

>> fittype('a*time^2+b*time+c','independent','time','dependent','height')
Undefined operator '==' for input arguments of type 'struct'.

Error in symvar (line 26)
[b,e] = findrun(s==' ');

Error in fittype>iCreateCustomFittype (line 463)
variableNames = symvar( obj.expr );

Error in fittype>iCreateFittype (line 351)
    obj = iCreateCustomFittype( obj, varargin{:} );

Error in fittype (line 328)
                obj = iCreateFittype( obj, varargin{:} );

I'll kep digging into the matlab code, but I'd appreciate if someone had an idea of what's going on...

Thanks

Answer 1

[Walter Roberson]

Try

restoredefaultpath

and then try again. If it works then you have a path problem; quit MATLAB, re-enter, and use pathtool to look at what your path is like and look for .m files in those that might be interfering.

[Giacomo Ciani]

Got it! Thanks so much! Now I have to figure out what is interfering, but at least I know where to look...

(and I will avoid commenting on why Matlab has to have this ridiculous problem...)