Strange behavior of array

[VlasovRoman via Digitalmars-d-learn]

I get it in dmd 2.068.2 and dmd 2.069-b2. I think, that this 
behavior some strange:

I have some code:
enum int m = 10;
enum int n = 5;

ubyte[m][n] array;
for(int x = 0; x < m; x++) {
        for(int y = 0; y < n; y++) {
                array[x][y] = cast(ubyte)(x + y);
        }       
}

In runtime i get range violation error. Helps to change the index 
when accessing the array. What I don't understand?

Thanks.