diff --git a/CHANGES.rst b/CHANGES.rst
index e602b1308..d8cf34ba3 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -6,7 +6,7 @@ There's a frood who really knows where his towel is.
 1.3b2 (unreleased)
 ^^^^^^^^^^^^^^^^^^
 
-- Use timestamp object instead of string of timestamp to represent image modified date (fixes `#686`_).
+- Fix ``TypeError`` when changing default image scale on basic tiles (fixes `#686`_).
   [rodfersou]
 
 - Fixed adding a 'more' link in list tiles.
diff --git a/src/collective/cover/tests/test_upgrades.py b/src/collective/cover/tests/test_upgrades.py
index 4c9a9de52..4069e245a 100644
--- a/src/collective/cover/tests/test_upgrades.py
+++ b/src/collective/cover/tests/test_upgrades.py
@@ -3,7 +3,6 @@
 from collective.cover.config import IS_PLONE_5
 from collective.cover.controlpanel import ICoverSettings
 from collective.cover.testing import INTEGRATION_TESTING
-from persistent.dict import PersistentDict
 from persistent.mapping import PersistentMapping
 from plone import api
 from plone.registry.interfaces import IRegistry
@@ -590,8 +589,9 @@ def test_registrations(self):
         self.assertGreaterEqual(int(version), int(self.to_version))
         self.assertEqual(self._how_many_upgrades_to_do(), 2)
 
-    def test_fix_image_modified_date(self):
-        title = u'Update all tiles to fix modified date'
+    def test_fix_image_field_modification_time(self):
+        from persistent.dict import PersistentDict
+        title = u'Fix image field modification time'
         step = self._get_upgrade_step(title)
         assert step is not None
 
@@ -614,7 +614,7 @@ def test_fix_image_modified_date(self):
         dmgr.annotations[dmgr.key] = PersistentDict(old_data)
 
         data = dmgr.get()
-        self.assertIsInstance(data['image_mtime'], str)
+        assert isinstance(data['image_mtime'], str)
 
         # run the upgrade step to validate the update
         self._do_upgrade_step(step)
diff --git a/src/collective/cover/upgrades/v15/__init__.py b/src/collective/cover/upgrades/v15/__init__.py
index ade549f55..045f3e0e4 100644
--- a/src/collective/cover/upgrades/v15/__init__.py
+++ b/src/collective/cover/upgrades/v15/__init__.py
@@ -5,8 +5,8 @@
 from plone.tiles.interfaces import ITileDataManager
 
 
-def fix_image_modified_date(context):
-    """Update all tiles to fix modified date"""
+def fix_image_field_modification_time(context):
+    """Fix image modification time to be float timestamp instead of string."""
 
     covers = context.portal_catalog(portal_type='collective.cover.content')
     logger.info('About to update {0} objects'.format(len(covers)))
diff --git a/src/collective/cover/upgrades/v15/configure.zcml b/src/collective/cover/upgrades/v15/configure.zcml
index 5c8d2bd01..78e047795 100644
--- a/src/collective/cover/upgrades/v15/configure.zcml
+++ b/src/collective/cover/upgrades/v15/configure.zcml
@@ -10,9 +10,9 @@
       profile="collective.cover:default">
 
     <genericsetup:upgradeStep
-        title="Update all tiles to fix modified date"
-        description="Modified date should be DateTime object instead of string of timestamp"
-        handler=".fix_image_modified_date"
+        title="Fix image field modification time"
+        description="Fix image modification time to be float timestamp instead of string."
+        handler=".fix_image_field_modification_time"
         />
 
     <genericsetup:upgradeStep