f='1/(sqrt(1+(x/9)^4)'作图出错

请教一个问题，先谢谢大家了
>> f='1/(sqrt(1+(x/9)^4)';
>> ezplot(f,[-10,10])
??? Error using ==> inlineeval
Error in inline expression ==> 1./(sqrt(1+(x./9).^4)
??? Error: Expression or statement is incorrect--possibly unbalanced (, {, or [.

Error in ==> inline.feval at 34
        INLINE_OUT_ = inlineeval(INLINE_INPUTS_, INLINE_OBJ_.inputExpr, INLINE_OBJ_.expr);

Error in ==> specgraph\private\ezplotfeval at 54
    z = feval(f,x(1));

Error in ==> ezplot>ezplot1 at 448
[y,f,loopflag] = ezplotfeval(f,x);

Error in ==> ezplot at 148
    [hp,cax] = ezplot1(cax,f{1},vars,labels,args{:});
是咋回事呢，为什么画不出图像来呢