Problem in mapping fragments starting at line nnn:All the key properties (xxxx.column) of the EntitySet xxxx must be mapped to all the key properties

Problem:

Error 3003: Problem in mapping fragments starting at line 257:All the key properties (table.column) of the EntitySet table must be mapped to all the key properties (table.column, table.column) of table table.

Solution:

Select the column giving you problems, hit F4 for properties, and change Nullable from (None) to False.