diff --git a/interp.py b/interp.py
index bec6528..9cb2b38 100644
--- a/interp.py
+++ b/interp.py
@@ -24,6 +24,13 @@
     def verb_Read(self, args):
         return Text(input())
 
+class InterpreterError(BaseException):
+    def __init__(self, error):
+        self.error = error
+
+    def __repr__(self):
+        return "InterpreterError(error '%s')" % (self.error)
+
 class Interpreter:
     def __init__(self, env):
         self.env = env
@@ -34,7 +41,7 @@
         elif value.__class__ == ast_types.Text:
             return Text(value.value)
         else:
-            print("Unimplemented value: %s" % (value))
+            raise InterpreterError("Unknown value type %s" % (value.__class__.__name__))
 
     def run_statement(self, ast):
         subject = self.resolve_value(ast.subject)
@@ -50,8 +57,7 @@
         return self.env[ast.subject]
 
     def run_conditional(self, ast):
-        print("Unimplemented conditional")
-        return None
+        raise InterpreterError("Conditionals are unimplemented")
 
     def run_command(self, ast):
         if ast.__class__ == ast_types.Statement:
@@ -61,8 +67,7 @@
         elif ast.__class__ == ast_types.Conditional:
             return self.run_conditional(ast)
         else:
-            print("Unknown command %s" % (ast))
-            return None
+            raise InterpreterError("Unknown command type %s" % (ast.__class__.__name__))
 
     def run(self, ast):
         ret = None
@@ -74,4 +79,8 @@
     env = {
         "System": Module_System(),
     }
-    return Interpreter(env).run(ast)
+    try:
+        return Interpreter(env).run(ast)
+    except InterpreterError as e:
+        print("Interpreter error: %s" % (e))
+        return None