diff --git a/astroplan/observer.py b/astroplan/observer.py
index f7f9b5e4..3abf2698 100644
--- a/astroplan/observer.py
+++ b/astroplan/observer.py
@@ -15,7 +15,6 @@
 from astropy.utils.compat.numpy import broadcast_to
 import astropy.units as u
 from astropy.time import Time
-from astropy.utils import isiterable
 import numpy as np
 import pytz
 
diff --git a/astroplan/scheduling.py b/astroplan/scheduling.py
index b4994e83..d9f6fb67 100644
--- a/astroplan/scheduling.py
+++ b/astroplan/scheduling.py
@@ -530,7 +530,10 @@ def _make_schedule(self, blocks):
         for b in blocks:
             if b.constraints is None:
                 b._all_constraints = self.constraints
+                b.constraints = [AltitudeConstraint(min=0 * u.deg)]
             else:
+                if AltitudeConstraint not in b.constraints:
+                    b.constraints.append(AltitudeConstraint(min=0 * u.deg))
                 b._all_constraints = self.constraints + b.constraints
             # to make sure the scheduler has some constraint to work off of
             # and to prevent scheduling of targets below the horizon
@@ -538,6 +541,9 @@ def _make_schedule(self, blocks):
             if b._all_constraints is None:
                 b._all_constraints = [AltitudeConstraint(min=0 * u.deg)]
                 b.constraints = [AltitudeConstraint(min=0 * u.deg)]
+                
+            # If there is no constraint on Altititude, specifically giving an AltitudeConstraint
+            # for the target to be above horizon.
             elif not any(isinstance(c, AltitudeConstraint) for c in b._all_constraints):
                 b._all_constraints.append(AltitudeConstraint(min=0 * u.deg))
                 if b.constraints is None:
diff --git a/docs/tutorials/scheduling.rst b/docs/tutorials/scheduling.rst
index 9d3ffeda..2541a139 100644
--- a/docs/tutorials/scheduling.rst
+++ b/docs/tutorials/scheduling.rst
@@ -172,7 +172,7 @@ Scheduling
 
 Now all we have left is to initialize the scheduler, input our list
 of blocks and the schedule to put them in. There are currently two
-schedulers to chose from in astroplan.
+schedulers to choose from in astroplan.
 
 The first is a sequential scheduler. It starts at the start_time and
 scores each block (constraints and target) at that time and then