Problem with curves and raytracing

I have created a bell like shape with the use of a bezier curve which was then converted to a mesh and then put into shape with the spin function.

When rendering like it is, it looks okay. The surface of the “bell” is flat and has a solid color.
But when applying raytracing transparency and rendering I get this:

I get something similar when applying subdivisions.

Is this normal or am I doing something wrong?

Any help would be highly appreciated…

looks like you have a texture to the alpha channel applied! go to the material buttons (F5), abd see if your material uses any textures

No the model uses no textures at all.
Just a simple material got applied to it.

can you post the .blend - file?

Enter Edit mode [TAB],
Select all vertices, Press [A] until all vertices turn Yellow,
Press Ctrl_N and confirm the prompt

Now try rendering.

That did the trick.

Thanks for the help !