diff --git a/bin/curvecalc b/bin/curvecalc
--- a/bin/curvecalc
+++ b/bin/curvecalc
@@ -146,17 +146,19 @@ class Subterm:
         make_attributes_from_args('submaster', 'subexpr')
     def scaled(self, t):
         subexpr_eval = self.subexpr.eval(t)
-        # this stops an error that didn't used to happen. i think when
-        # the expr is empty, subexpr_eval becomes None, so this zero
-        # is ok
-        if subexpr_eval is None:
-            subexpr_eval = 0
-        if isinstance(subexpr_eval, Submaster.Submaster):
-            # if the expression returns a submaster, just return it
-            return subexpr_eval
-        else:
-            # otherwise, return our submaster multiplied by the value returned
-            return self.submaster * subexpr_eval
+        # we prevent any exceptions from escaping, since they cause us to
+        # stop sending levels
+        try:
+            if isinstance(subexpr_eval, Submaster.Submaster):
+                # if the expression returns a submaster, just return it
+                return subexpr_eval
+            else:
+                # otherwise, return our submaster multiplied by the value 
+                # returned
+                return self.submaster * subexpr_eval
+        except Exception, e:
+            dispatcher.send("expr_error", sender=self.subexpr, exc=str(e))
+            return Submaster.Submaster('Error: %s' % str(e), temporary=True)
 
 class Subtermview(tk.Frame):
     def __init__(self,master,st,**kw):