Draw long-section of M-string with a loop in it

My master string loops back on itself and when I draw it in long-section it shows as just a squiggly (technical term) line bunched up at the left side of the drawing. Strangely enough, section strings through ground and additional design models are drawn correctly.

I tried changing the offsets in 017 but to no avail.

The long-section wizard draws it properly, as does the vertical alignment option, so I'm guessing there's something in my code.

DRAW    TMP FIETSPAD
017GRADGRAD
800                   0
803LONGNOPATRUN                                 500                          50
804LONG    M5Z0
826M5Z0M5Z0
999
DISPLAY
FINISH

Any clues?

S