C++ :: Two Dimensional Pointer To Object?

I am migrating a scientific code from Java to C++.

What's wrong with the two functions? I can use the int** like a two dimensional array but not the Agent**.

I receive this error: "No operator = matches this operand".

(I have allocated two dimensional arrays for objectSpace and agentSpace somewhere else).

Code:
int** objectSpace;
Agent** agentSpace;
void Space::removeAgentAt(Point p) {
agentSpace[p.x][p.y] = NULL;
}
void Space::putAgentTo(Agent agent, Point newP) {
agentSpace[newP.x][newP.y] = agent;
}