Unclosed shapes in pocket ignored

[Lokoman]

I imported a simple dxf file from OnShape and have looked at each individual point to make sure there were no gaps but cannot get my head around this one..  I have read the responses about doing a zero fillet and have done a polyline-remove overlaps but can't get past this error. 

[dh42]

Hello

To do a pocket you must use a closed polyline, not a bunch of simple lines as you did. (the simple lines must be joined)

Have a look on this tuto

https://cambamcnc.com/forum/index.php?topic=1624.msg9100#msg9100

The corrected file is in attachment.

++
David