请教非线性拟合问题！！

diamondyang

function nnn
x=[-2.018,-1.818,-1.236,-1.018,-0.855,-0.018,0.291,0.691];
y=[1.025,1.161,1.193,1.064,1.139,1.232,1.309,1.428];
beta0=[5 0.5];
[beta,r,J]=nlinfit(x,y,'volum',beta0); %非线性拟合函数的系数、残差
nlintool(x,y,'volum',beta0) %拟合曲线图
beta
plot(x,y,'o')

function yy=volum(beta,x)   
yy=beta(1)./(1+(beta(1)/1.139-1)*exp(-beta(2)*(x+0.855)));
上面是我编的非线性拟合的M文件，x和y是拟合数据，yy为拟合公式。运行后出现如下错误：

??? Error using ==> nlinfit
The model function 'volum' generated the following error:
Error using ==> volum
Too many input arguments.
Error in ==> nnn at 5
[beta,r,J]=nlinfit(x,y,'volum',beta0); 非线性拟合函数的系数、残差

请大家帮忙解决！谢谢