diff --git a/agents.py b/agents.py
index 047eb3fd6..ff049aa4a 100644
--- a/agents.py
+++ b/agents.py
@@ -85,10 +85,10 @@ def __init__(self, program=None):
         self.bump = False
         self.holding = []
         self.performance = 0
-        if program is None:
+        if program is None or not isinstance(program, collections.Callable):
+            print("Can't find a valid program for {}, falling back to default.".format(self.__class__.__name__))
             def program(percept):
                 return eval(input('Percept={}; action? '.format(percept)))
-        assert isinstance(program, collections.Callable)
         self.program = program
 
     def can_grab(self, thing):
@@ -297,12 +297,14 @@ def add_thing(self, thing, location=None):
         for it. (Shouldn't need to override this."""
         if not isinstance(thing, Thing):
             thing = Agent(thing)
-        assert thing not in self.things, "Don't add the same thing twice"
-        thing.location = location if location is not None else self.default_location(thing)
-        self.things.append(thing)
-        if isinstance(thing, Agent):
-            thing.performance = 0
-            self.agents.append(thing)
+        if thing in self.things:
+            print("Can't add the same thing twice")
+        else:
+            thing.location = location if location is not None else self.default_location(thing)
+            self.things.append(thing)
+            if isinstance(thing, Agent):
+                thing.performance = 0
+                self.agents.append(thing)
 
     def delete_thing(self, thing):
         """Remove a thing from the environment."""
diff --git a/tests/test_agents.py b/tests/test_agents.py
index 0162a78b8..699e317f7 100644
--- a/tests/test_agents.py
+++ b/tests/test_agents.py
@@ -1,4 +1,5 @@
 from agents import Direction
+from agents import Agent
 from agents import ReflexVacuumAgent, ModelBasedVacuumAgent, TrivialVacuumEnvironment
 
 
@@ -65,3 +66,9 @@ def test_ModelBasedVacuumAgent() :
     # check final status of the environment
     assert environment.status == {(1,0):'Clean' , (0,0) : 'Clean'}
 
+def test_Agent():
+    def constant_prog(percept):
+        return percept
+    agent = Agent(constant_prog)
+    result = agent.program(5)
+    assert result == 5