"snapshot too large" error when initializing logical replication (9.4)

I sometimes get the error "snapshot too large" from my logical 
replication walsender process when in response to a CREATE_REPLICATION_SLOT.

This is in SnapBuildExportSnapshot in snapbuild.c

newxcnt is 212 at that point

I have max_connections = 200

procArray->maxProcs=212

Should we be testing
newxcnt > GetMaxSnapshotXidCount()

instead of
newxcnt >= GetMaxSnapshotXidCount()