Cannot change the type of a mullion created prior to release 4.0 to a corner mullion type. Try deleting the existing mullion and placing a new one.

public static FailureDefinitionId CannotCreateOldCornerMullion { get; }

Public Shared ReadOnly Property CannotCreateOldCornerMullion As FailureDefinitionId
	Get

public:
static property FailureDefinitionId^ CannotCreateOldCornerMullion {
	FailureDefinitionId^ get ();
}