diff --git a/content/treestyletab/res/tabsDragUtils.js b/content/treestyletab/res/tabsDragUtils.js
index 562c536e..e498276b 100644
--- a/content/treestyletab/res/tabsDragUtils.js
+++ b/content/treestyletab/res/tabsDragUtils.js
@@ -13,9 +13,12 @@
 
  original:
    http://github.com/piroor/fxaddonlib-tabs-drag-utils
+
+ depends on:
+   https://github.com/clear-code/js-extended-immutable
 */
 (function() {
-	const currentRevision = 36;
+	const currentRevision = 37;
 
 	if (!('piro.sakura.ne.jp' in window)) window['piro.sakura.ne.jp'] = {};
 
@@ -34,6 +37,9 @@
 	const Ci = Components.interfaces;
 	const TAB_DROP_TYPE = 'application/x-moz-tabbrowser-tab';
 
+	// fix path to extended-immutable.js
+	var { ExtendedImmutable } = Components.utils.import('resource://treestyletab-modules/lib/extended-immutable.js', {});
+
 	var tabsDragUtils = {
 		revision : currentRevision,
 
@@ -142,16 +148,26 @@
 			this.updatedTabDNDObservers.push(aObserver);
 
 			if (typeof aObserver._getDropEffectForTabDrag === 'function' &&
-				aObserver._getDropEffectForTabDrag.toSource().indexOf('tabsDragUtils') < 0) {
-				let original = aObserver._getDropEffectForTabDrag;
-				aObserver.__TabsDragUtils_original__getDropEffectForTabDrag = original;
-				eval('aObserver._getDropEffectForTabDrag = '+
-					original.toSource().replace(
-						'dt.mozItemCount > 1',
-						'$& && !window["piro.sakura.ne.jp"].tabsDragUtils.isTabsDragging(arguments[0])'
-					)
-				);
-				aObserver.__TabsDragUtils_updated__getDropEffectForTabDrag = aObserver._getDropEffectForTabDrag;
+				!aObserver.__tabsDragUtils__getDropEffectForTabDrag) {
+				aObserver.__tabsDragUtils__getDropEffectForTabDrag = aObserver._getDropEffectForTabDrag;
+				aObserver._getDropEffectForTabDrag = function(aEvent, ...aArgs) {
+					var fakeItemCount = window["piro.sakura.ne.jp"].tabsDragUtils.isTabsDragging(aEvent) ? 1 : null ;
+					var fakeDataTransfer = new ExtendedImmutable(aEvent.dataTransfer, {
+						get mozItemCount() {
+							if (fakeItemCount) {
+								fakeItemCount = null;
+								return fakeItemCount;
+							}
+							return aEvent.dataTransfer.mozItemCount;
+						}
+					});
+					var fakeEvent = new ExtendedImmutable(aEvent, {
+						get dataTransfer() {
+							return fakeDataTransfer;
+						}
+					});
+					return aObserver.__tabsDragUtils__getDropEffectForTabDrag(fakeEvent, ...aArgs);
+				};
 			}
 
 			if ('_animateTabMove' in aObserver &&
diff --git a/modules/lib/extended-immutable.js b/modules/lib/extended-immutable.js
new file mode 100644
index 00000000..cd7ff3c7
--- /dev/null
+++ b/modules/lib/extended-immutable.js
@@ -0,0 +1,80 @@
+// Original author: ClearCode Inc.
+// License: MIT
+// Repository: https://github.com/clear-code/js-extended-immutable
+
+var EXPORTED_SYMBOLS = ['ExtendedImmutable'];
+
+function ExtendedImmutable(aImmutable, aExtraProperties) {
+  var descriptors = {};
+  Object.keys(aExtraProperties).forEach(function(aName) {
+    descriptors[aName] = Object.getOwnPropertyDescriptor(aExtraProperties, aName);
+  });
+
+  var base = Object.create({}, descriptors);
+  return new Proxy(base, {
+    getPrototypeOf: function(aTarget) {
+      return Object.getPrototypeOf(aImmutable);
+    },
+    setPrototypeOf: function(aTarget, aPrototype) {
+      return Object.setPrototypeOf(aImmutable, aPrototype);
+    },
+    isExtensible: function(aTarget) {
+      return Object.isExtensible(aImmutable);
+    },
+    preventExtensions: function(aTarget) {
+      return Object.preventExtensions(aImmutable);
+    },
+    getOwnPropertyDescriptor: function(aTarget, aProperty) {
+      return Object.getOwnPropertyDescriptor(aImmutable, aProperty);
+    },
+    defineProperty: function(aTarget, aProperty, aDescriptor) {
+      return Object.defineProperty(aImmutable, aProperty, aDescriptor);
+    },
+    has: function(aTarget, aProperty) {
+      return aProperty in aImmutable;
+    },
+    get: function(aTarget, aName, aReceiver) {
+      if (descriptors.hasOwnProperty(aName)) {
+        let value;
+        if (typeof descriptors[aName].get == 'function')
+          value = descriptors[aName].get.call(base);
+        else
+          value = descriptors[aName].value;
+
+        if (typeof value == 'function')
+          return value.bind(base);
+        else
+          return value;
+      }
+      var value = aImmutable[aName];
+      if (typeof value == 'function')
+        return value.bind(aImmutable);
+      else
+        return value;
+    },
+    set: function(aTarget, aName, aValue, aReceiver) {
+      if (descriptors.hasOwnProperty(aName)) {
+        if (typeof descriptors[aName].set == 'function')
+          return descriptors[aName].set.call(base, aValue);
+        else
+          return descriptors[aName].value = aValue;
+      }
+      return aImmutable[aName] = aValue;
+    },
+    deleteProperty: function(aTarget, aProperty) {
+      delete aImmutable[aProperty];
+    },
+    enumerate: function(aTarget) {
+      return Reflect.enumerate(aImmutable);
+    },
+    ownKeys: function(aTarget) {
+      return Object.getOwnPropertyNames(aImmutable);
+    },
+    apply: function(aTarget, aThis, aArgs) {
+      return aImmutable.apply(aThis, aArgs);
+    },
+    construct: function(aTarget, aArgs) {
+      return new aImmutable(...aArgs);
+    }
+  });
+}