Carry over values from Custom fields to System Default Fields

Carry over values from Custom fields to System Default Fields

When I try and convert a lead in to a potential, it gives me a few default options like "Amount", "Potential Name", Closing Date", "Status" and "Contact Role". 

I've managed to equate the amount field from Leads and carry it over to Potential. However, I would like to do the same for potential name. In our case, a potential is a project, its not a company. So, I created a Text field in Leads with the name "Potential Name' wherein a user would fill the project name. Upon conversion that value should be seen in the "Potential Name" under the Potential Module. 

But, field mapping does not allow me to map the "Potential Name" from Leads to "Potential Name" under potential even though both are text based fields. 

Any suggestions?