diff --git a/graphite_api/finders/whisper.py b/graphite_api/finders/whisper.py
index e643006..feffa21 100644
--- a/graphite_api/finders/whisper.py
+++ b/graphite_api/finders/whisper.py
@@ -122,6 +122,9 @@ def fetch(self, startTime, endTime):  # noqa
             if isinstance(cached_datapoints, dict):
                 cached_datapoints = cached_datapoints.items()
             for timestamp, value in sorted(cached_datapoints):
+                # filter only to cached datapoints within [start, end)
+                if not (timestamp >= start and timestamp < end):
+                    continue
                 interval = timestamp - (timestamp % step)
                 i = int(interval - start) // step
                 values[i] = value
@@ -133,7 +136,7 @@ class GzippedWhisperReader(WhisperReader):
     def get_intervals(self):
         fh = gzip.GzipFile(self.fs_path, 'rb')
         try:
-            info = whisper.__readHeader(fh)  # evil, but necessary.
+            info = getattr(whisper, '__readHeader')(fh)  # evil, but necessary.
         finally:
             fh.close()