flatten says 'Expecting MAP, encountered LONG' for a string

[Totktonada]

avro = require('avro_schema')
schema = {type = 'record', name = 'foo', fields = {{name = 'bar', type = 'string'}}}
ok, h = avro.create(schema)
ok, m = avro.compile(h)
m.flatten('hello')
---
- false
- Expecting MAP, encountered LONG