Bulk Api Custom Data Objects Mapping to Accounts

We are trying to create a custom objects and map it to an account using the bulk REST api.  For contacts, we are able to accomplish this using the following:

"{{CustomObject[48].Contact.Field(C_EmailAddress)}}"

For accounts we tried this:

"{{{{CustomObject[48].Account.Field(M_CustomAccount_ID)}}}}"

However we get the following error:

"failures":[{"field":"account_mapping_field","stackTrace":[{"field":"fields"}],"value":"{{CustomObject[73].Account.Field(M_CustomAccount_ID)}}","constraint":"

Must not contain Account fields (since linking is not enabled for imports)."}]}

Any suggestions?