diff --git a/requirements.txt b/requirements.txt
index 80af3c7b7a2d2..b81e1789484b4 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -17,6 +17,7 @@ geopy==1.11.0
 gunicorn==19.8.0
 humanize==0.5.1
 idna==2.6
+isodate==0.6.0
 markdown==2.6.11
 pandas==0.23.1
 parsedatetime==2.0.0
diff --git a/setup.py b/setup.py
index 8db1597c0c537..38c39a8921428 100644
--- a/setup.py
+++ b/setup.py
@@ -75,6 +75,7 @@ def get_git_sha():
         'gunicorn',  # deprecated
         'humanize',
         'idna',
+        'isodate',
         'markdown',
         'pandas>=0.18.0',
         'parsedatetime',
diff --git a/superset/migrations/versions/3dda56f1c4c6_migrate_num_period_compare_and_period_.py b/superset/migrations/versions/3dda56f1c4c6_migrate_num_period_compare_and_period_.py
index c2a0d9af27962..fef8bb81a11d2 100644
--- a/superset/migrations/versions/3dda56f1c4c6_migrate_num_period_compare_and_period_.py
+++ b/superset/migrations/versions/3dda56f1c4c6_migrate_num_period_compare_and_period_.py
@@ -125,7 +125,7 @@ def upgrade():
     session = db.Session(bind=bind)
 
     for chart in session.query(Slice):
-        params = json.loads(chart.params)
+        params = json.loads(chart.params or '{}')
 
         if not params.get('num_period_compare'):
             continue
@@ -150,7 +150,7 @@ def downgrade():
     session = db.Session(bind=bind)
 
     for chart in session.query(Slice):
-        params = json.loads(chart.params)
+        params = json.loads(chart.params or '{}')
 
         if 'time_compare' in params or 'comparison_type' in params:
             params.pop('time_compare', None)