Are you passing something like thing => undef to your constructor?

package Bit::Vector {
    use Moose;
}

package Bit::Thing {
    use Moose;
    has thing => (
        isa     => "Bit::Vector",
        is      =>"rw",
        default => sub { return Bit::Vector->new },
    );
}

Bit::Thing->new;  # fine, "thing" gets the default value
Bit::Thing->new(thing => undef);  # croaks
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