This is clearly a bug and I fixed it. The bug fix will be available in the next version.
This happens when the abscissa (X) of start and end points of a wall are almost the same. In fact, they even appear equal in wall modification dialog, but are different in memory. As you can see in the file attached to this message, this can be fixed in the current version with a copy/paste of the X value of start point in the X value of end point in the wall dialog.

Thank you for your help. [:)]