OSX Mail crashing

My Mail is crashing regularly after any actions are taken, say maybe ten seconds after using. This error:

Application Specific Information:

*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[__NSDictionaryM setObject:forKeyedSubscript:]: key cannot be nil'

terminating with uncaught exception of type NSException

abort() called

Crashed Thread: 4 Dispatch queue: NSOperationQueue 0x60800082ab60 (QOS: UNSPECIFIED)

Exception Type: EXC_CRASH (SIGABRT)

Exception Codes: 0x0000000000000000, 0x0000000000000000

Exception Note: EXC_CORPSE_NOTIFY

Any suggestions?