Access violation with OnDrawCell Procedure in delphi

Question

I'm quite new at programming, and I'm having trouble with an Access Violation in the DrawCell Procedure below.

I'm creating a maze and each cell in the 2D Array maze has a boolean attribute called "wall". When the program starts, all cells in the array are set to be a "Wall"

the access violation occurs at the line

if maze[ACol, ARow].Wall = true then




procedure TfrmMazeGame.StrGridMazeDrawCell(Sender: TObject; ACol, ARow: 
integer; Rect: TRect; State: TGridDrawState);
begin
    if maze[ACol, ARow].Wall = true then
    begin
        StrGridMaze.canvas.Brush.Color := clGreen
    end
    else if maze[ACol, ARow].Wall = false then
        StrGridMaze.canvas.Brush.Color := clblue;
    StrGridMaze.canvas.fillrect(Rect);
end;