Convex Mesh::load Convex Hull: Convex hull init failed

I use Unity with 2DToolKit to develop a 2d game. Now I encounter a problem: I add several block sprites(irregular polygon) and set their Collider Type as Polygon, When I select the option of "Convex", console shows error information: Convex Mesh::load Convex Hull: Convex hull init failed. After I refer to official document about "Mesh Collider" and know "Convex: If enabled, this Mesh Collider will collide with other Mesh Colliders. Convex Mesh Colliders are limited to 255 triangles." . Maybe Convex Mesh Colliders exceed 255 triangles in mine. But how can I have a collision detection with irregular polygon in a right method ?