diff --git a/zipline/algorithm.py b/zipline/algorithm.py
index b6507e30..c8753614 100644
--- a/zipline/algorithm.py
+++ b/zipline/algorithm.py
@@ -21,8 +21,10 @@ from pandas.tseries.tools import normalize_date
 import numpy as np
 
 from datetime import datetime
-
 from itertools import groupby, chain, repeat
+from numbers import Integral
+from operator import attrgetter
+
 from six.moves import filter
 from six import (
     exec_,
@@ -30,7 +32,6 @@ from six import (
     itervalues,
     string_types,
 )
-from operator import attrgetter
 
 
 from zipline.errors import (
@@ -96,6 +97,7 @@ from zipline.utils.events import (
     TimeRuleFactory,
 )
 from zipline.utils.factory import create_simulation_parameters
+from zipline.utils.functional import is_a
 from zipline.utils.math_utils import tolerant_equals
 from zipline.utils.preprocess import preprocess
 
@@ -608,8 +610,7 @@ class TradingAlgorithm(object):
             if isinstance(identifier, Asset):
                 asset = self.asset_finder.retrieve_asset(sid=identifier.sid,
                                                          default_none=True)
-
-            elif hasattr(identifier, '__int__'):
+            elif isinstance(identifier, Integral):
                 asset = self.asset_finder.retrieve_asset(sid=identifier,
                                                          default_none=True)
             if asset is None:
@@ -618,6 +619,12 @@ class TradingAlgorithm(object):
         self.trading_environment.write_data(
             equities_identifiers=identifiers_to_build)
 
+        # We need to clear out any cache misses that were stored while trying
+        # to do lookups.  The real fix for this problem is to not construct an
+        # AssetFinder until we `run()` when we actually have all the data we
+        # need to so.
+        self.asset_finder._reset_caches()
+
         return self.asset_finder.map_identifier_index_to_sids(
             identifiers, as_of_date,
         )
diff --git a/zipline/assets/assets.py b/zipline/assets/assets.py
index 69a1cc17..5a2f85fa 100644
--- a/zipline/assets/assets.py
+++ b/zipline/assets/assets.py
@@ -99,12 +99,23 @@ class AssetFinder(object):
         #
         # The caches are read through, i.e. accessing an asset through
         # retrieve_asset will populate the cache on first retrieval.
-        self._asset_cache = {}
-        self._asset_type_cache = {}
+        self._caches = (self._asset_cache, self._asset_type_cache) = {}, {}
 
         # Populated on first call to `lifetimes`.
         self._asset_lifetimes = None
 
+    def _reset_caches(self):
+        """
+        Reset our asset caches.
+
+        You probably shouldn't call this method.
+        """
+        # This method exists as a workaround for the in-place mutating behavior
+        # of `TradingAlgorithm._write_and_map_id_index_to_sids`.  No one else
+        # should be calling this.
+        for cache in self._caches:
+            cache.clear()
+
     def lookup_asset_types(self, sids):
         """
         Retrieve asset types for a list of sids.