def is_valid_matrix(mtx):
    """
    >>> is_valid_matrix([[1, 2, 3], [4, 5, 6], [7, 8, 9]])
    True
    >>> is_valid_matrix([[1, 2, 3], [4, 5], [7, 8, 9]])
    False
    """
    if mtx == []:
        return False

    cols = len(mtx[0])

    for col in mtx:
        if len(col) != cols:
            return False

    return True