diff --git a/nc_time_axis/__init__.py b/nc_time_axis/__init__.py
index 0899e86..d4620a6 100644
--- a/nc_time_axis/__init__.py
+++ b/nc_time_axis/__init__.py
@@ -231,7 +231,7 @@ def default_units(cls, sample_point, axis):
             if isinstance(sample_point, np.ndarray):
                 sample_point = sample_point.reshape(-1)
             calendars = np.array([point.calendar for point in sample_point])
-            if np.all(calendars[0] == calendars):
+            if np.all(calendars == calendars[0]):
                 calendar = calendars[0]
             else:
                 raise ValueError('Calendar units are not all equal.')
diff --git a/requirements.txt b/requirements.txt
index 0fc4191..8caa35a 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,4 +1,4 @@
-matplotlib
+matplotlib<2
 mock
 netcdf4>=1.1
 numpy