How to fit a defined function?

I want to fit a custom function to my experimental data. For simplicity, I have some arbitary x & y values and a very simple linear function. I write the following code:

clear all;

x = [1 2 3 4 5]'; % x data
y = [.8 4 10 18 23]'; % y data

Y = lsqcurvefit(fun,1,x,y) % fitting function 'fun' defined below to find parameter 'a'

function y = fun(a,x)
y = a*x; % just a simple function for example, in actual problem, it is a long complicated function with various parameters
end

I get the following error: 'Not enough input arguements.'

I know this particular simple function can be defined as anonymous function and be fitted but I don't it that way.