diff --git a/packages/blaze/builtins.js b/packages/blaze/builtins.js
index 18e165c09..21fcd21eb 100644
--- a/packages/blaze/builtins.js
+++ b/packages/blaze/builtins.js
@@ -1,5 +1,5 @@
 Blaze._calculateCondition = function (cond) {
-  if (cond instanceof Array && cond.length === 0)
+  if (HTML.isArray(cond) && cond.length === 0)
     cond = false;
   return !! cond;
 };
diff --git a/packages/html-tools/parse.js b/packages/html-tools/parse.js
index 109e9fa2f..7b3296327 100644
--- a/packages/html-tools/parse.js
+++ b/packages/html-tools/parse.js
@@ -187,7 +187,7 @@ getContent = HTMLTools.Parse.getContent = function (scanner, shouldStopFunc) {
         // as in `FOO.apply(null, content)`.
         if (content == null)
           content = [];
-        else if (! (content instanceof Array))
+        else if (! HTML.isArray(content))
           content = [content];
 
         items.push(HTML.getTag(tagName).apply(
diff --git a/packages/htmljs/html.js b/packages/htmljs/html.js
index ce758772d..55c8adac2 100644
--- a/packages/htmljs/html.js
+++ b/packages/htmljs/html.js
@@ -171,10 +171,7 @@ Raw.prototype.htmljsType = Raw.htmljsType = ['Raw'];
 
 
 HTML.isArray = function (x) {
-  // could change this to use the more convoluted Object.prototype.toString
-  // approach that works when objects are passed between frames, but does
-  // it matter?
-  return (x instanceof Array);
+  return x instanceof Array || Array.isArray(x);
 };
 
 HTML.isConstructedObject = function (x) {
@@ -185,10 +182,22 @@ HTML.isConstructedObject = function (x) {
   // if you assign to a prototype when setting up the class as in:
   // `Foo = function () { ... }; Foo.prototype = { ... }`, then
   // `(new Foo).constructor` is `Object`, not `Foo`).
-  return (x && (typeof x === 'object') &&
-          (x.constructor !== Object) &&
-          (typeof x.constructor === 'function') &&
-          (x instanceof x.constructor));
+  if(!x || (typeof x !== 'object')) return false;
+  // Is this a plain object?
+  let plain = false;
+  if(Object.getPrototypeOf(x) === null) {
+    plain = true;
+  } else {
+    let proto = x;
+    while(Object.getPrototypeOf(proto) !== null) {
+      proto = Object.getPrototypeOf(proto);
+    }
+    plain = Object.getPrototypeOf(x) === proto;
+  }
+
+  return !plain &&
+    (typeof x.constructor === 'function') &&
+    (x instanceof x.constructor);
 };
 
 HTML.isNully = function (node) {