使用GetSplitCurves报错？请指点。

plaer

Polyline pcb;
    ...
    
    Point3dCollection clipboundary=new Point3dCollection();
    for(int j = 0; j < pcb.NumberOfVertices ; j++)
    {
     clipboundary.Add(pcb.GetPoint3dAt(j));
    }
    DBObjectCollection dbc;
    Curve c =ent as Curve;
    if(c!=null)
       dbc=c.GetSplitCurves(clipboundary);


GetSplitCurves(clipboundary) 处报错：
************** Exception Text **************
System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> Autodesk.AutoCAD.Runtime.Exception: eInvalidInput
   at Autodesk.AutoCAD.Runtime.Interop.ThrowExceptionForErrorStatus(Int32 errorStatus)
   at Autodesk.AutoCAD.DatabaseServices.Curve.GetSplitCurves(Point3dCollection points)
   at MyCommand2006.ObjectClass.Crop(ObjectId oid) in d:\autocad开发学习\mysource\mycommand2006\objectclass.cs:line 366
   at MyCommand2006.MyCommand.gettype() in d:\autocad开发学习\mysource\mycommand2006\mycommand2006.cs:line 82
   --- End of inner exception stack trace ---
   at System.RuntimeMethodHandle._InvokeMethodFast(Object target, Object[] arguments, SignatureStruct& sig, MethodAttributes methodAttributes, RuntimeTypeHandle typeOwner)
   at System.RuntimeMethodHandle.InvokeMethodFast(Object target, Object[] arguments, Signature sig, MethodAttributes methodAttributes, RuntimeTypeHandle typeOwner)
   at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture, Boolean skipVisibilityChecks)
   at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
   at AcMgCommandClass.Invoke(AcMgCommandClass* , gcroot<System::Reflection::MethodInfo __gc \*>* mi)
   at Autodesk.AutoCAD.Runtime.SEHExceptionFilter.InvokeWorker()
   at Autodesk.AutoCAD.Runtime.ExceptionFilter.Invoke()