diff --git a/js/src/dataflow.js b/js/src/dataflow.js
index af06642..f8f3aeb 100644
--- a/js/src/dataflow.js
+++ b/js/src/dataflow.js
@@ -9,6 +9,7 @@ function Graph() {
   this.edgesReverse = Immutable.Map();
   this.damagedNodes = Immutable.Set();
   this.currentSubjectId = null;
+  this.enforceSubjectPresence = true;
   this.observablePropertyCounter = 0;
 }
 
@@ -30,6 +31,8 @@ Graph.prototype.recordObservation = function (objectId) {
   if (this.currentSubjectId) {
     this.edgesForward = MapSet.add(this.edgesForward, objectId, this.currentSubjectId);
     this.edgesReverse = MapSet.add(this.edgesReverse, this.currentSubjectId, objectId);
+  } else if (this.enforceSubjectPresence) {
+    throw new Error('Attempt to observe ' + objectId + ' with no currentSubjectId');
   }
 };
 
diff --git a/js/test/test-dataflow.js b/js/test/test-dataflow.js
index 8cea7c2..d025e22 100644
--- a/js/test/test-dataflow.js
+++ b/js/test/test-dataflow.js
@@ -50,6 +50,7 @@ describe('dataflow edges, damage and subjects', function () {
 describe('DerivedCell', function () {
   describe('simple case', function () {
     var g = new Dataflow.Graph();
+    g.enforceSubjectPresence = false;
     var c = DerivedCell(g, 'c', function () { return 123; });
     var d = DerivedCell(g, 'd', function () { return c.value * 2; });
     it('should be properly initialized', function () {
@@ -78,6 +79,7 @@ describe('DerivedCell', function () {
 
   describe('a more complex case', function () {
     var g = new Dataflow.Graph();
+    g.enforceSubjectPresence = false;
 
     function add(a, b) { return a + b; }
     var xs = new Cell(g, Immutable.List.of(1, 2, 3, 4), 'xs');