substituting variables for meshgrid variables

Good Day,

For my studies I wanted to substitue some symbols, it works fine but the following line doesn't work:

u = 0:0.2:5;
v = u;
[U,V] = meshgrid(u,v);
subs((a1.*y.^2)./(1+y.^2)-x,{x,y},{U,V});

if i substitute the variables x and y with "normal vectors it works fine but not with the meshgrid:

??? Error using ==> horzcat
CAT arguments dimensions are not consistent.

Error in ==> sym.subs>@(x)[char(x),','] at 246
    s = cellfun(@(x)[char(x) ','],X,'UniformOutput',false);

Error in ==> sym.subs>getNames at 246
    s = cellfun(@(x)[char(x) ','],X,'UniformOutput',false);

Error in ==> sym.subs>tryFunctionHandle at 190
xvarnames = getNames(X);

Error in ==> sym.subs>mupadsubs at 137
[G,worked] = tryFunctionHandle(F,X,Y);

Error in ==> sym.subs at 125
G = mupadsubs(F,X,Y);

Could anyone help me what I need to change?