Got it to work; override this.GetShapeDefinition() = seq{ let def = new PolygonDef() def.VertexCount <- 3 def.Vertices.[0] <- new Vector2(1.0f, 2.0f) def.Vertices.[1] <- new Vector2(-2.0f,-3.0f) def.Vertices.[2] <- new Vector2(2.0f,-3.0f) def.Restitution <- 0.0f def.Friction <- 1.0f def.Density <- 1.0f yield (def :> ShapeDef) }