This error is a very common error which will occur at the time of initialising the model.
in your case it might have occurred if your existing table and the model definition have not matched check on to it.
If you really feel it complex check on to this "http://www.codeproject.com/Articles/482801/DatabaseplusFirstplusDevelopmentpluswithplusASP-NE" which creates a model for your existing table