lsqnonlin stops iteration with an error

hi, i have a problem with lsqnonlin. i want to fit experimental data to the solution of a differential equation via least square. lsqonline seems to work correctly, but then it stops and i have no idea why. hopefully you can give me a hint. Thats the error message:

     4         20        0.291191      0.0338188          0.234            0
     5         24        0.260773      0.0747579          0.284            0

     ??? Error using ==> snls at 390
     The number of equations must be greater than n.

     Error in ==> lsqncommon at 149
    [xC,FVAL,LAMBDA,JACOB,EXITFLAG,OUTPUT,msgData]=...

    Error in ==> lsqnonlin at 241
    [xCurrent,Resnorm,FVAL,EXITFLAG,OUTPUT,LAMBDA,JACOB] = ...

    Error in ==> lsqZH at 32
    [x_top, resnorm_top] = lsqnonlin(fun_top,x0,lb,ub, options);

My Code looks like this:

    options = optimset('Display','iter');

    fun_top=@(x)lsqFunctionZH(x, z_exp, r_exp);

    lb = [-1.0, 0.01, 1.4];
    ub = [-0.5, 1.0, s_top];
    x0 = [-0.75, 0.1, 1.45];

    [x_top, resnorm_top] = lsqnonlin(fun_top,x0,lb,ub, options);

In my fun_top.m i solve the differential equation and return the vector r_sol-r_exp. For some x, the equation cant be solved, so the function returns NaN. Is that a problem?

Thank you for your help.

[Andsalot]

Maybe i solved this problem. I forgot to return a NaN-vector with the length of r_exp, if the differential equation cant be solved.