diff --git a/resources/idlharness.js b/resources/idlharness.js
index 039a6bba96cddc1..be072ff618b83f9 100644
--- a/resources/idlharness.js
+++ b/resources/idlharness.js
@@ -679,6 +679,7 @@ IdlArray.prototype.is_json_type = function(type)
        case "Uint32Array":
        case "Uint8ClampedArray":
        case "Float32Array":
+       case "Float64Array":
        case "ArrayBuffer":
        case "DataView":
        case "any":
@@ -1207,8 +1208,18 @@ IdlArray.prototype.assert_type_is = function(value, type)
             assert_regexp_match(value, /^([\x00-\ud7ff\ue000-\uffff]|[\ud800-\udbff][\udc00-\udfff])*$/);
             return;
 
+        case "Int8Array":
+        case "Int16Array":
+        case "Int32Array":
+        case "Uint8Array":
+        case "Uint16Array":
+        case "Uint32Array":
+        case "Uint8ClampedArray":
+        case "Float32Array":
+        case "Float64Array":
+        case "ArrayBuffer":
         case "DataView":
-            assert_equals(typeof value, "DataView");
+            assert_true(value instanceof self[type], "wrong type: not a " + type);
             return;
 
         case "object":