Virtual site impossible permutations error

[SimonBoothroyd]

Describe the bug

I'm seeing an impossible permutations error when trying to assign a simple divalent v-site to molecules similar to the example shown in this repro case.

To Reproduce

from openff.toolkit.topology import Molecule
from openff.toolkit.typing.engines.smirnoff import VirtualSiteHandler
from simtk import unit

molecule = Molecule.from_mapped_smiles("[H:4][C:1]1([N:2]=[N:3]1)[H:5]")

vsite_handler = VirtualSiteHandler(version="0.3")
vsite_handler.add_parameter(
    parameter=VirtualSiteHandler.VirtualSiteDivalentLonePairType(
        type="DivalentLonePair",
        smirks="[*:1]~[#7X2:2]~[*:3]",
        name="EP",
        distance=0.4 * unit.angstrom,
        outOfPlaneAngle=0.0 * unit.degrees,
        charge_increment=[0.0, 0.0, 0.0] * unit.elementary_charge,
        sigma=0.0 * unit.angstrom,
        epsilon=0.0 * unit.kilojoules_per_mole,
        match="once",
    )
)

matches = vsite_handler.find_matches(molecule.to_topology())
print(matches)

Output

Traceback (most recent call last):
  File "/xxx/repro.py", line 30, in <module>
    main()
  File "/xxx/repro.py", line 25, in main
    matches = vsite_handler.find_matches(molecule.to_topology())
  File "/xxx/openff/toolkit/typing/engines/smirnoff/parameters.py", line 5609, in find_matches
    return self._find_matches(
  File "/xxx/openff/toolkit/typing/engines/smirnoff/parameters.py", line 5445, in _find_matches
    index_of_key = tdc.index_of(key, possible=orders)
  File "/xxx/openff/toolkit/topology/topology.py", line 152, in index_of
    assert all([p in permutations for p in possible]), (
AssertionError: Possible permutations [(0, 1, 2), (2, 1, 0), (0, 2, 1), (1, 2, 0)] is impossible!

Computing environment (please complete the following information):

• Operating system OSX
• Output of running conda list

openff-toolkit-base       0.10.3
openeye-toolkits          2021.2.0

Additional context