Failed to merge Leads [26066185]

[Sarin_Nair1]

I'm trying to merge 2 leads and I'm getting this system error message (attached). There are 2 leads with the same email id, one is Marketo lead and the other is a CRM lead. Does anyone know?

[Gerard_van_den_]

could it be that the SFDC one is a Contact, not a Lead?

[Dan_Stevens_]

SFDC or MS Dynamics CRM?

[Sarin_Nair1]

SFDC