diff --git a/bin/effecteval b/bin/effecteval
--- a/bin/effecteval
+++ b/bin/effecteval
@@ -53,6 +53,7 @@ def newEnvelopeCurve(graph, ctx, uri, la
 
 @inlineCallbacks
 def insertEnvelopePoints(curve):
+    # wrong: we might not be adding to the currently-playing song.
     musicStatus = yield getMusicStatus()
     songTime=musicStatus['t']
     songDuration=musicStatus['duration']
diff --git a/light9/effecteval/effect.py b/light9/effecteval/effect.py
--- a/light9/effecteval/effect.py
+++ b/light9/effecteval/effect.py
@@ -133,7 +133,11 @@ class EffectNode(object):
         for c in self.codes:
             codeNs = ns.copy()
             codeNs.update(c.pyResources)
-            lineOut = eval(c.expr, codeNs)
+            try:
+                lineOut = eval(c.expr, codeNs)
+            except Exception as e:
+                e.expr = c.expr
+                raise e
             ns[c.outName] = lineOut
         if 'out' not in ns:
             log.error("ran code for %s, didn't make an 'out' value", self.uri)
diff --git a/light9/effecteval/effectloop.py b/light9/effecteval/effectloop.py
--- a/light9/effecteval/effectloop.py
+++ b/light9/effecteval/effectloop.py
@@ -147,6 +147,8 @@ class EffectLoop(object):
             except Exception as exc:
                 now = time.time()
                 if now > self.lastErrorLog + 5:
+                    if hasattr(exc, 'expr'):
+                        log.error('in expression %r', exc.expr)
                     log.error("effect %s: %s" % (e.uri, exc))
                     self.lastErrorLog = now
         log.debug('eval %s effects, got %s outputs', len(self.currentEffects), len(outputs))