"Can't Cast Hash to Date" when trying .update

[wolfkarl]

It seems that I can't update date fields in the parent class:

if @contract.update(contract_params) ...

gets me

*TypeError in ContractsController#update_contract *
can't cast Hash to date

StackTrace:

activerecord (4.2.4) lib/active_record/connection_adapters/abstract/quoting.rb:34:in `rescue in type_cast'
activerecord (4.2.4) lib/active_record/connection_adapters/abstract/quoting.rb:23:in `type_cast'
activerecord (4.2.4) lib/active_record/connection_adapters/sqlite3_adapter.rb:290:in `block in exec_query'
activerecord (4.2.4) lib/active_record/connection_adapters/sqlite3_adapter.rb:289:in `map'
activerecord (4.2.4) lib/active_record/connection_adapters/sqlite3_adapter.rb:289:in `exec_query'
activerecord (4.2.4) lib/active_record/connection_adapters/abstract/database_statements.rb:76:in `exec_insert'
activerecord (4.2.4) lib/active_record/connection_adapters/abstract/database_statements.rb:108:in `insert'
activerecord (4.2.4) lib/active_record/connection_adapters/abstract/query_cache.rb:14:in `insert'
activerecord (4.2.4) lib/active_record/relation.rb:64:in `insert'
activerecord (4.2.4) lib/active_record/persistence.rb:524:in `_create_record'
...

Things work fine if I

• use the same code but only change things for the parent element itsself
• don't try to update the date (e.g. by removing it from permitted parameters)

Any ideas?