diff --git a/bin/weewx/restx.py b/bin/weewx/restx.py
index 639c58ff..dcbd0813 100644
--- a/bin/weewx/restx.py
+++ b/bin/weewx/restx.py
@@ -647,20 +647,6 @@ class CachedValues(object):
                 elif packet['usUnits'] != self.unit_system:
                     raise ValueError("Mixed units encountered in cache. %s vs %s" % \
                                      (self.unit_sytem, packet['usUnits']))
-            elif packet[k] is None:
-                # FIXME: the cache should not check for values of None.
-                # however, if we blindly accept None as a possible value, then
-                # any partial packet will break the cache.  the fix probably
-                # requires changes to the StdWXCalculate service and the
-                # drivers themselves.  if a driver knows a sensor has a bad
-                # value, only then it should use a value of None.  if a driver
-                # sends partial packets, it should send only the observed
-                # values, not a full packet with None in the unobserved fields.
-                # similarly for calculate service.  if the service does not
-                # have all the inputs for a given derived, it should not insert
-                # a derived with value of None.  it should insert a value of
-                # None only if all the inputs exist and the result is None.
-                continue
             else:
                 # cache each value, associating it with the it was cached
                 self.values[k] = {'value': packet[k], 'ts': ts}
diff --git a/docs/changes.txt b/docs/changes.txt
index 308213d1..c48a2b79 100644
--- a/docs/changes.txt
+++ b/docs/changes.txt
@@ -16,6 +16,11 @@ buffer).  Thanks to users Markus Biewer and Cameron.  Fixes issue #288.
 Added automatic clearing of logger for CC3000 driver to prevent logger
 overflow (the CC3000 logger is not a circular buffer).
 
+Parameterize the configuration directory in weewx-multi init script.
+
+In StdWXCalculate, use None for an observation only if the variables on which
+the derived depends are available and None.  Fixes issue #291.
+
 3.8.0 11/22/2017
 
 The `stats.py` example now works with heating and cooling degree days.