convert lead to contact facing issues

convert lead to contact facing issues

I want to convert a lead to contact.

Using this URL:
 https://crm.zoho.com/crm/private/json/Leads/convertLead?authtoken=myauthtoken&scope=crmapi&leadId=2847269000000418119&xmlData=

XML Data:
<Potentials><row no="1"><option val="createPotential">false</option><option val="assignTo"> mymailid@gmail.com</option><option val="notifyLeadOwner">true</option><option val="notifyNewEntityOwner">true</option></row></Potentials>

Response got from server:
{"response": {"uri":"/crm/private/json/Leads/convertLead","error": {"code":4487,"message":"Given lead id is invalid or already been deleted"}}}

The lead id passed in URL isn't deleted and also valid id which I've copied from address bar of browser. Why I'm getting this message. Please help.

Thanks,
Guna.P