how to call MultiScalarField2D

Hi everybody!
How the heck can I call MultiScalarField2D.
I construct it for example as the following:
MultiScalarField2D plk(2,2);
When I try to assign a value to plk[1,1], it wont be compiled!
plk[1,1] = 5.4;

For other data types I do not have this problem.
For these below, it works.
vector viscosities(5);
viscosities[3] = 0.63;
Array<double,3> velocity(0., 0., 0.5);
velocity[0] = 0.1;

Please tell me why.
many thanks