Fix error when Z is never mentionned in GCode

printrun/gcoder.py

@@ -468,8 +468,8 @@ class GCode(object):
                     ymin = min(ymin, line.current_y)
                     ymax = max(ymax, line.current_y)
 
-        all_zs = self.all_zs
-        zmin = min(zmin, min(all_zs))
+        all_zs = self.all_zs.union(set([zmin]))
+        zmin = min(all_zs)
         zmax = max(all_zs)
 
         self.xmin = xmin if not math.isinf(xmin) else 0