@tomleslie 

yes indeed - what's not to like!

Thanks for an interesting comment that - for me at least - has shed quite a lot of light on using plot3d.

Thanks - the qoutes did the trick.

Thank you for drawing my attention to piecewise. It seems to address my problem much more accurate. But alas - no plot, and the strange error message still pops up, even when I try your proposal in a completely new document.