Some circular arc constructors return an empty geometry or an error message where they should return a valid degenerate arc geometry. A degenerate arc geometry means that either ICircularArc::IsLine or ICircularArc::IsPoint return TRUE. One example of where this problem occurs is for the ConstructTangentRadiusAngle method. In this method, if the radius is entered as zero, no message is returned and an empty geometry results.
This is a known problem.
ArcGIS Service Pack 1 improves the circular arc constructors to always generate degenerate circular arc geometries where possible. See Related Information below.