Index: Objects/abstract.c
===================================================================
--- Objects/abstract.c	(revision 82887)
+++ Objects/abstract.c	(working copy)
@@ -2508,7 +2508,12 @@
         if (retval == 0) {
             PyObject *c = PyObject_GetAttr(inst, __class__);
             if (c == NULL) {
-                PyErr_Clear();
+                if(PyErr_ExceptionMatches(PyExc_AttributeError)) {
+                    PyErr_Clear();
+                }
+                else {
+                    retval = -1;
+                }
             }
             else {
                 if (c != (PyObject *)(inst->ob_type) &&
@@ -2526,8 +2531,12 @@
             return -1;
         icls = PyObject_GetAttr(inst, __class__);
         if (icls == NULL) {
-            PyErr_Clear();
-            retval = 0;
+            if(PyErr_ExceptionMatches(PyExc_AttributeError)) {
+                PyErr_Clear();
+            }
+            else {
+                retval = -1;
+            }
         }
         else {
             retval = abstract_issubclass(icls, cls);
Index: Lib/test/test_isinstance.py
===================================================================
--- Lib/test/test_isinstance.py	(revision 82887)
+++ Lib/test/test_isinstance.py	(working copy)
@@ -81,7 +81,21 @@
 
         self.assertRaises(TypeError, isinstance, I(), C())
 
+    # check that we don't mask non AttributeErrors
+    # see: http://bugs.python.org/issue1574217
+    def test_isinstance_dont_mask_non_attribute_error(self):
+        class C(object):
+            def getclass(self):
+                raise RuntimeError()
+            __class__=property(getclass)
 
+        c=C()
+        self.assertRaises(RuntimeError, isinstance, c, bool)
+
+        # test another code path
+        class D: pass
+        self.assertRaises(RuntimeError, isinstance, c, D)
+
 
 # These tests are similar to above, but tickle certain code paths in
 # issubclass() instead of isinstance() -- really PyObject_IsSubclass()