Strange error after adding a table to Entity Model

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entity-framework

I have a customised dynamic data application based on an Entity Framework model. The application compiles and runs fine, until I open the model and add a single, simple table. As soon as I add this table, I get the following error on all my tables:
Must specify mapping for all key properties.
Key properties are and have been properly mapped since November 2009, so what could happen during the adding of one little three column table to invalidate my whole model?