diff --git a/src/BTrees/_datatypes.py b/src/BTrees/_datatypes.py
index 5035144..904f6f8 100644
--- a/src/BTrees/_datatypes.py
+++ b/src/BTrees/_datatypes.py
@@ -178,6 +178,13 @@ class O(KeyDataType):
         if type(item) is object: # pylint:disable=unidiomatic-typecheck
             raise TypeError("Can't use object() as keys")
 
+        # Check for type objects (classes); they have default comparison
+        # based on address which is not suitable for BTree keys
+        if type(item) is type:
+            raise TypeError(
+                "Type objects cannot be used as BTree keys"
+            )
+
         # Now more complicated checks to be sure we didn't
         # inherit default comparison on any version of Python.
 
@@ -208,6 +215,11 @@ class O(KeyDataType):
                 # TODO: Shouldn't we only check __cmp__ on Python 2?
                 # Python 3 won't use it.
                 and getattr(item, '__cmp__', None) is None):
+            cls_name = getattr(item, '__name__', None)
+            if cls_name is not None:
+                raise TypeError(
+                    "Object of class %s has default comparison" % cls_name
+                )
             raise TypeError("Object has default comparison")