Plot a fourier transform of a 1x1 sym. WARNING: The following error was reported evaluating the function in FunctionLine update: Unable to convert expression into double array.

I have a function "J" which depends by the variable "theta".

In particular I have obtained a 1x1 sym value. I can plot this function (the interval is from 0 to 0.6) with fplot that is:

fplot(J, [0 0.6])

It is all ok since I obtain the correct graph.

Now I want to evaluate its Fourier transform. Since J is a 1x1 sym, I used the code "fourier" and not "fft", that is:

Y=fourier(J);

I obtain again a 1X1 sym.

Now I want to plot Y in order to obtain its spectrum, so:

fplot(abs(Y));

Note that I am not considering the normalization, but at the moment it is not the problem since I need only a way to plot a 1x1 sym value.

This is the message:

Warning: Function behaves unexpectedly on array inputs. To improve performance, properly vectorize your function to return an output with the
same size and shape as the input arguments. 
> In matlab.graphics.function.FunctionLine>getFunction
  In matlab.graphics.function.FunctionLine/updateFunction
  In matlab.graphics.function.FunctionLine/set.Function_I
  In matlab.graphics.function.FunctionLine/set.Function
  In matlab.graphics.function.FunctionLine
  In fplot>singleFplot (line 232)
  In fplot>@(f)singleFplot(cax,{f},limits,extraOpts,args) (line 191)
  In fplot>vectorizeFplot (line 191)
  In fplot (line 161)
  In grafico_delle_densita_di_correnti_theta_unilatero (line 139)
  
  Warning: Error updating FunctionLine.

The following error was reported evaluating the function in FunctionLine update: Unable to convert expression into double array.