MATLAB: Fimplicit gives a warning ”Function behaves unexpectedly on array inputs.”

I just wanna plot a dispersion equation according to the paper.

here is my code, really simple.

clc,clear all,

fimplicit(@(x,y) 9*y^4-((1-9)*9+2*9^2*10*(1-cos(x)))*y^2+2*9^2*10*(1-cos(x)),[-3 3 0 20]);

But the plot is not accurate near 0. What is going on and how can I fix it？

Warning 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.ImplicitFunctionLine>getFunction

In matlab.graphics.function.ImplicitFunctionLine/updateFunction

In matlab.graphics.function.ImplicitFunctionLine/set.Function_I

In matlab.graphics.function.ImplicitFunctionLine/set.Function

In matlab.graphics.function.ImplicitFunctionLine

In fimplicit>singleFimplicit (line 185)

In fimplicit>@(f)singleFimplicit(cax,f,limits,extraOpts,args) (line 148)

In fimplicit>vectorizeFimplicit (line 148)

In fimplicit (line 126)

In Test (line 2)