diff --git a/scrapy_httpcache/extensions/httpcache_storage.py b/scrapy_httpcache/extensions/httpcache_storage.py
index ec3dfa6..0b804ef 100644
--- a/scrapy_httpcache/extensions/httpcache_storage.py
+++ b/scrapy_httpcache/extensions/httpcache_storage.py
@@ -88,7 +88,7 @@ def _read_data(self, spider, request):
                 returnValue()
             else:
                 return
-        if 0 < self.expiration_secs < (datetime.datetime.now - data['update_time']).seconds:
+        if 0 < self.expiration_secs < (datetime.datetime.now() - data['update_time']).seconds:
             yield self._coll.delete_one({'fingerprint': key})
             if six.PY2:
                 returnValue()
diff --git a/setup.py b/setup.py
index ed70fba..9059a38 100644
--- a/setup.py
+++ b/setup.py
@@ -40,7 +40,8 @@
         'Topic :: Software Development :: Libraries :: Python Modules',
     ],
     install_requires=[
-        'scrapy>=1.4.0'
+        'scrapy>=1.4.0',
+        'txmongo==18.1.0'
     ],
     extras_require=extras_require,
 )