This is Null from one member function to another

I do have to following code:

bool SudokuField::blockContainsByBlock(const short &block_x, const short &block_y, const short &value)
{
    assertRange(block_x, 0, 2);
    assertRange(block_y, 0, 2);
    return this->blockContains(block_x * 3, block_y * 3, value);
}

bool SudokuField::blockContains(const short &x, const short &y, const short &value)
{
    assertRange(value, 1, 9);
    assertRange(x, 0, 8);
    assertRange(y, 0, 8);
    const short startX = (x / 3) * 3;
    const short startY = (y / 3) * 3;
    for (short offset = 0; offset < 9; offset++)
    {
        if (this->field[startX + (offset % 3)][startY + (offset / 3)] == value)
        {
            return true;
        }
    }
    return false;
}

and I do run into Segmentationfault Errors. While debugging the blockContainsByBlock function does have a valid this address(eg. 0x778b30). When it calls the blockContains member function the this(inside blockContains) is suddenly a 0x0 address and the code crashes right at the first line. I can provide more code if needed but that is everything that crashes. The parameters are set correctly on each function.