diff --git a/cachier/mongo_core.py b/cachier/mongo_core.py
index a1aaa20..2d2077d 100644
--- a/cachier/mongo_core.py
+++ b/cachier/mongo_core.py
@@ -7,9 +7,11 @@
 # http://www.opensource.org/licenses/MIT-license
 # Copyright (c) 2016, Shay Palachy <shaypal5@gmail.com>
 
+import sys  # to make sure that pymongo was imported
 import pickle  # for serialization of python objects
 from datetime import datetime
 import time   # to sleep when waiting on Mongo cache\
+import warnings  # to warn if pymongo is missing
 
 try:
     from pymongo import (
@@ -20,9 +22,6 @@ try:
     from bson.binary import Binary  # to save binary data to mongodb
 except ImportError:  # pragma: no cover
     pass
-    # warnings.warn((
-    #     "Cachier warning: pymongo was not found. "
-    #     "MongoDB cores will not work."))
 
 from .base_core import _BaseCore
 
@@ -39,6 +38,10 @@ class _MongoCore(_BaseCore):
     _INDEX_NAME = 'func_1_key_1'
 
     def __init__(self, mongetter, stale_after, next_time):
+        if 'pymongo' not in sys.modules:
+            warnings.warn((
+                "Cachier warning: pymongo was not found. "
+                "MongoDB cores will not function."))
         _BaseCore.__init__(self, stale_after, next_time)
         self.mongetter = mongetter
         self.mongo_collection = self.mongetter()
diff --git a/tests/test_pickle_core.py b/tests/test_pickle_core.py
index 6fdee6b..896f31b 100644
--- a/tests/test_pickle_core.py
+++ b/tests/test_pickle_core.py
@@ -48,7 +48,8 @@ def test_pickle_core():
     _takes_5_seconds.clear_cache()
 
 
-DELTA = timedelta(seconds=3)
+SECONDS_IN_DELTA = 3
+DELTA = timedelta(seconds=SECONDS_IN_DELTA)
 
 
 @cachier(stale_after=DELTA, next_time=False)
@@ -85,9 +86,10 @@ def test_stale_after_next_time():
     val3 = _stale_after_next_time(1, 3)
     assert val1 == val2
     assert val1 != val3
-    sleep(3)
+    sleep(SECONDS_IN_DELTA + 1)
     val4 = _stale_after_next_time(1, 2)
     assert val4 == val1
+    sleep(0.5)
     val5 = _stale_after_next_time(1, 2)
     assert val5 != val1
     _stale_after_next_time.clear_cache()