diff --git a/.gitignore b/.gitignore
index c424dc3..e80d19d 100644
--- a/.gitignore
+++ b/.gitignore
@@ -16,3 +16,4 @@ coverage.xml
 *.egg
 dist
 .eggs/
+.dir-locals.el
diff --git a/BTrees/_base.py b/BTrees/_base.py
index 032ea62..1a6b838 100644
--- a/BTrees/_base.py
+++ b/BTrees/_base.py
@@ -883,11 +883,16 @@ def _set(self, key, value=None, ifunset=False):
                 max_size = self.max_leaf_size
             if child.size > max_size:
                 self._grow(child, index)
-        elif (grew is not None and
-              child.__class__ is self._bucket_type and
-              len(data) == 1 and
-              child._p_oid is None
-              ):
+
+        # If a BTree contains only a single bucket, BTree.__getstate__()
+        # includes the bucket's entire state, and the bucket doesn't get
+        # an oid of its own.  So if we have a single oid-less bucket that
+        # changed, it's *our* oid that should be marked as changed -- the
+        # bucket doesn't have one.
+        if (grew is not None and
+            child.__class__ is self._bucket_type and
+            len(data) == 1 and
+            child._p_oid is None):
             self._p_changed = 1
         return result
 
diff --git a/BTrees/tests/common.py b/BTrees/tests/common.py
index b689901..28eb33a 100644
--- a/BTrees/tests/common.py
+++ b/BTrees/tests/common.py
@@ -768,7 +768,6 @@ def testPop(self):
         # Too many arguments.
         self.assertRaises(TypeError, t.pop, 1, 2, 3)
 
-
 class BTreeTests(MappingBase):
     # Tests common to all BTrees
 
@@ -1091,6 +1090,56 @@ def testDamagedIterator(self):
                 self.assertEqual(str(v), str(k[0]))
         self._checkIt(t)
 
+    def testAddTwoSetsChanged(self):
+        # A bug in the BTree Python implementation once
+        # caused adding a second item to a tree to fail
+        # to set _p_changed (adding the first item sets it because
+        # the _firstbucket gets set, but the second item only grew the
+        # existing bucket)
+        t = self._makeOne()
+        # Note that for the property to actually hold, we have to fake a
+        # _p_jar and _p_oid
+        t._p_oid = b'\0\0\0\0\0'
+        class Jar(object):
+            def __init__(self):
+                self._cache = self
+                self.registered = None
+
+            def mru(self, arg):
+                pass
+            def readCurrent(self, arg):
+                pass
+            def register(self, arg):
+                self.registered = arg
+
+        t._p_jar = Jar()
+        t[1] = 3
+        # reset these, setting _firstbucket triggered a change
+        t._p_changed = False
+        t._p_jar.registered = None
+        t[2] = 4
+        self.assertTrue(t._p_changed)
+        self.assertEqual(t, t._p_jar.registered)
+
+        # Setting the same key to a different value also triggers a change
+        t._p_changed = False
+        t._p_jar.registered = None
+        t[2] = 5
+        self.assertTrue(t._p_changed)
+        self.assertEqual(t, t._p_jar.registered)
+
+        # Likewise with only a single value
+        t = self._makeOne()
+        t._p_oid = b'\0\0\0\0\0'
+        t._p_jar = Jar()
+        t[1] = 3
+        # reset these, setting _firstbucket triggered a change
+        t._p_changed = False
+        t._p_jar.registered = None
+
+        t[1] = 6
+        self.assertTrue(t._p_changed)
+        self.assertEqual(t, t._p_jar.registered)
 
 class NormalSetTests(Base):
     # Test common to all set types
diff --git a/CHANGES.rst b/CHANGES.rst
index a9cc90e..454632a 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -7,6 +7,9 @@
 - Suppress testing 64-bit values in OLBTrees on 32 bit machines.
   See:  https://github.com/zopefoundation/BTrees/issues/9
 
+- Fix _p_changed for small pure-Python BTrees.
+  See   https://github.com/zopefoundation/BTrees/issues/11
+
 
 4.1.1 (2014-12-27)
 ------------------