diff --git a/src/lib/number_box.py b/src/lib/number_box.py
index 419fbc0..343928c 100644
--- a/src/lib/number_box.py
+++ b/src/lib/number_box.py
@@ -709,7 +709,9 @@ class NumberBox (Gtk.DrawingArea):
         pass
 
     def set_value (self, v):
-        if 0 < v <= self.upper:
+        if v == None:
+            self.set_text('')
+        elif 0 < v <= self.upper:
             self.set_text(str(v))
         else:
             self.set_text('')
