diff --git a/packages/observe-sequence/observe_sequence.js b/packages/observe-sequence/observe_sequence.js
index 687bf4882f..769f75e93c 100644
--- a/packages/observe-sequence/observe_sequence.js
+++ b/packages/observe-sequence/observe_sequence.js
@@ -9,6 +9,8 @@ var warn = function () {
   }
 };
 
+function isArray(arr) {return arr instanceof Array || _.isArray(arr)}
+
 var idStringify = MongoID.idStringify;
 var idParse = MongoID.idParse;
 
@@ -94,7 +96,7 @@ ObserveSequence = {
 
         if (!seq) {
           seqArray = seqChangedToEmpty(lastSeqArray, callbacks);
-        } else if (seq instanceof Array || _.isArray(seq)) {
+        } else if (isArray(seq)) {
           seqArray = seqChangedToArray(lastSeqArray, seq, callbacks);
         } else if (isStoreCursor(seq)) {
           var result /* [seqArray, activeObserveHandle] */ =
@@ -126,7 +128,7 @@ ObserveSequence = {
   fetch: function (seq) {
     if (!seq) {
       return [];
-    } else if (seq instanceof Array || _.isArray(seq)) {
+    } else if (isArray(seq)) {
       return seq;
     } else if (isStoreCursor(seq)) {
       return seq.fetch();
diff --git a/packages/observe-sequence/observe_sequence_tests.js b/packages/observe-sequence/observe_sequence_tests.js
index f746636cec..c82bb17dce 100644
--- a/packages/observe-sequence/observe_sequence_tests.js
+++ b/packages/observe-sequence/observe_sequence_tests.js
@@ -147,6 +147,14 @@ Tinytest.add('observe-sequence - initial data for all sequence types', function
     {addedAt: ["37", {_id: "37", bar: 2}, 1, null]}
   ]);
 
+  // sub-classed arrays
+  runOneObserveSequenceTestCase(test, function () {
+    return new ArraySubclass({_id: "13", foo: 1}, {_id: "37", bar: 2});
+  }, function () {}, [
+    {addedAt: ["13", {_id: "13", foo: 1}, 0, null]},
+    {addedAt: ["37", {_id: "37", bar: 2}, 1, null]}
+  ]);
+
   runOneObserveSequenceTestCase(test, function () {
     var coll = new Mongo.Collection(null);
     coll.insert({_id: "13", foo: 1});
@@ -609,7 +617,7 @@ Tinytest.add('observe-sequence - subclassed number arrays', function (test) {
     dep.depend();
     return seq;
   }, function () {
-    seq = [1, 3, 2, 3];
+    seq = new ArraySubclass(1, 3, 2, 3);
     dep.changed();
   }, [
     {addedAt: [1, 1, 0, null]},