What this error means: “The dependent element in the namespace has incomplete content

Question

The 'Dependent' element in the namespace ' http://schemas.microsoft.com/ado/2008/09/edm ' has incomplete content. List of expected items: "PropertyRef" in the namespace http://schemas.microsoft.com/ado/2008/09/edm '.

First I use EF4, db. The error is due to the association in the edmx file, unfortunately, I am not very versed in EF Database First (I used the code before), but this error really makes no sense.