Salesforce lookup relationship with null value

Hi,

I have written a lookup relationship in mule to upsert the record.

abc__r: {

type: "abc__c",

Name: (payload01.value )

},

It worked fine when there is value in relationship (Parent table). But the value is not available it throws error.

Foreign key external ID: xxxxxxx not found for field Name in entity Abc__c",

Is it possible to insert null if value not found (like Dataloader.io)

Regards,

Saurin