SQLite claims duplicate rows on insert whereas none can be found

I have a table in a SQLite database created with the code below. Note the compound primary key:

db.create_table(:person_hash) do
  Integer :person_id
  Bignum :hash // MD5 hash in hex stored as numeric: hash.to_i(16)
  primary_key [:person_id, :hash]
end

This table has some rows already:

puts db[:person_hash].where(:person_id => 285577).all
# {:person_id=>285577, :hash=>306607097659338192312932577746542919680}

Now, when I try to insert this:

db[:person_hash].insert({:person_id=>285577, :hash=>306607097659338206333361532286405644297})

I get this:

SQLite3::ConstraintException: columns person_id, hash are not unique (Sequel::DatabaseError)

If the row does not already exist in the table, how can it be a duplicate?

I tried inserting another hash for the same person ID instead, and it worked without problems.