[NTG-context] Some math symbols fail in MetaPost in MKIV

Hi, as it seems, some symbols in math mode seem to fail when put inside MetaPost. In the example I tried, both "~" and "|" were causing processing errors when used inside MetaPost (I tried both btex...etex as well as \sometxt). It works fine in MKII however. The minimal example is attached to this mail. Best Regards, Andreas.