uassert( 10149,
                         "Invalid mod field name, may not end in a period",
                         fieldName[ strlen( fieldName ) - 1 ] != '.' );

If strlen( fieldName ) == 0, this will read a character before the beginning of fieldName. Because fieldName will point to a field name within a bson element, this will not cause undefined or incorrect behavior currently. But we might want to clean it up a bit.

Test to trigger

c = db.c;
c.drop();

c.save( {} );
c.update( {}, { $set:{ '':4 } } );
printjson( c.findOne() );