Can't create Corner Mullion due to the angle between the neighboring panels. Try changing the Parameters in the Curtain Wall Mullion Family.

Namespace: Autodesk.Revit.DB
Assembly: RevitAPI (in RevitAPI.dll) Version: 2015.0.0.0 (2015.0.0.0)

Syntax

C#
`public static FailureDefinitionId CannotCreateCornerMullionDueAngle { get; }`

Visual Basic
`Public Shared ReadOnly Property CannotCreateCornerMullionDueAngle As FailureDefinitionId Get`

Visual C++
`public: static property FailureDefinitionId^ CannotCreateCornerMullionDueAngle { FailureDefinitionId^ get (); }`

See Also