3 files changed, 21 insertions, 2 deletions
diff --git a/src/zope/interface/_zope_interface_coptimizations.c b/src/zope/interface/_zope_interface_coptimizations.c
index a6ef2dc..7eaf219 100644
--- a/src/zope/interface/_zope_interface_coptimizations.c
+++ b/src/zope/interface/_zope_interface_coptimizations.c
@@ -149,7 +149,7 @@ getObjectSpecification(PyObject *ignored, PyObject *ob)
   PyObject *cls, *result;
 
   result = PyObject_GetAttr(ob, str__provides__);
-  if (result != NULL)
+  if (result != NULL && PyObject_TypeCheck(result, &SpecType))
     return result;
 
   PyErr_Clear();
diff --git a/src/zope/interface/declarations.py b/src/zope/interface/declarations.py
index 3bc28a7..44a40e2 100644
--- a/src/zope/interface/declarations.py
+++ b/src/zope/interface/declarations.py
@@ -31,6 +31,7 @@ __docformat__ = 'restructuredtext'
 import sys
 import weakref
 from zope.interface.interface import InterfaceClass, Specification
+from zope.interface.interface import SpecificationBase
 from ro import mergeOrderings, ro
 import exceptions
 from types import ClassType, ModuleType
@@ -1248,7 +1249,8 @@ def getObjectSpecification(ob):
 
     provides = getattr(ob, '__provides__', None)
     if provides is not None:
-        return provides
+        if isinstance(provides, SpecificationBase):
+            return provides
 
     try:
         cls = ob.__class__
diff --git a/src/zope/interface/tests/test_declarations.py b/src/zope/interface/tests/test_declarations.py
index 8831009..de09f1c 100644
--- a/src/zope/interface/tests/test_declarations.py
+++ b/src/zope/interface/tests/test_declarations.py
@@ -402,6 +402,23 @@ def test_picklability_of_implements_specifications():
     
     """
 
+def test_provided_by_with_slots():
+    """
+
+    This is an edge case: if the __slots__ of a class contain '__provides__',
+    using providedBy() on that class should still work (this occurs, for
+    example, when providing an adapter for a concrete class.)
+
+    >>> import zope.interface
+    >>> class Slotted(object):
+    ...     __slots__ = ('__provides__')
+    >>> class IFoo(zope.interface.Interface):
+    ...     pass
+    >>> IFoo.providedBy(Slotted)
+    False
+
+    """
+
 
 def test_suite():
     suite = unittest.TestSuite()