Memory access violation

Function CLIP_ClipPoly(POLY_Clip.CLIP_Poly, Interpolate=True)	

	Local POLY_Input.CLIP_Poly, POLY_Output.CLIP_Poly, POLY_Temp.CLIP_Poly, POLY_Swap.CLIP_Poly   
	Local LOOP_Plane.CLIP_Plane
	Local LOOP_Vertex
	Local V1_Index, V2_Index, VI_Index
	Local V1_Inside, V2_Inside
	Local Vertices

	; Create a polygon to hold the new polygon that results each time we clip the input polygon.
		POLY_Temp = New CLIP_Poly
	
	; Set up the input/output buffer pointers.	
		POLY_Input  = POLY_Clip
		POLY_Output = POLY_Temp	
				
	; For each clipping plane...	
	For LOOP_Plane = Each CLIP_Plane
				
		; Clear all vertcies from the output polygon.
		POLY_Output\Vertices = 0
				
		; For each edge in the input polygon...
		For LOOP_Vertex = 0 To POLY_Input\Vertices-1

			; Caclulate the vertices that form this edge.

				V1_Index = LOOP_Vertex
				V2_Index = LOOP_Vertex+1
	
				If V2_Index = POLY_Input\Vertices Then V2_Index = 0
				
			; Are these vertices on the back side of the clipping plane?
			V1_Inside = CLIP_Vertex_Inside_Plane(POLY_Input, V1_Index, LOOP_Plane)
			V2_Inside = CLIP_Vertex_Inside_Plane(POLY_Input, V2_Index, LOOP_Plane)
		
			Select True

				; Output V2.			
				Case ((V1_Inside=True) And (V2_Inside=True))
				
					; Add V2 (endpoint of edge) to output polygon.
					CLIP_CopyVertex(POLY_Input, V2_Index, POLY_Output)


				; Output intersection point.	
				Case ((V1_Inside=True) And (V2_Inside=False))

					; Find the point at which the edge intersects this plane.
					CLIP_Edge_Intersect_Plane(POLY_Input, V1_Index, V2_Index, LOOP_Plane)
					
					; Add intersection point.
					Select Interpolate 
						Case True  CLIP_Add_Interpolated_Vertex(POLY_Output, V1_Index, V2_Index, CLIP_Intersect_U#)
						Case False CLIP_AddVertex(POLY_Output, CLIP_Intersect_X#, CLIP_Intersect_Y#, CLIP_Intersect_Z#)
					End Select


				; No output.	
				Case ((V1_Inside=False) And (V2_Inside=False))
				
					; Both vertcies are on the outside side of the plane. 
					; Don't add any vertices to the polygon.
			
				
				; Output Intersection point and V2.
				Case ((V1_Inside=False) And (V2_Inside=True))

					; Find the point at which the edge intersects this plane.
					CLIP_Edge_Intersect_Plane(POLY_Input, V1_Index, V2_Index, LOOP_Plane)

					; Add intersection point.
					Select Interpolate 
						Case True  CLIP_Add_Interpolated_Vertex(POLY_Output, V1_Index, V2_Index, CLIP_Intersect_U#)
						Case False CLIP_AddVertex(POLY_Output, CLIP_Intersect_X#, CLIP_Intersect_Y#, CLIP_Intersect_Z#)
					End Select

					; Add V2.
					CLIP_CopyVertex(POLY_Input, V2_Index, POLY_Output)
															
			End Select 		

		Next
		
		; If there are no vertices in the final polygon...
		If POLY_Output\Vertices = 0
			
			; The polygon has been completely clipped away.
						
			; Delete all vertices from the original polygon, delete the temporary storage buffer, and exit the function early.
			POLY_Clip\Vertices = 0
			Delete POLY_Temp  ; <- Memory access violation???
			Return
			
		EndIf	

		; Swap the pointers to the input polygon data and the output polygon data
		; This makes the output data from the last loop the input data to the next,
		; and uses the old input location to store the new output data.
		POLY_Swap   = POLY_Output
		POLY_Output = POLY_Input
		POLY_Input  = POLY_Swap
		
	Next	

	; If we've made it this far, we have a clipped polygon that lies within the frustum!

	; Copy the final output over the original polygon data, delete the temporary storage buffer, and exit the function.
	; (We swapped input and output at the end of the last loop, so input really points to the final output.)
	CLIP_CopyPoly(POLY_Input, POLY_Clip)
	Delete POLY_Temp
	Return 
	
		
End Function