diff --git a/Lib/test/test_isinstance.py b/Lib/test/test_isinstance.py
--- a/Lib/test/test_isinstance.py
+++ b/Lib/test/test_isinstance.py
@@ -82,7 +82,21 @@ class TestIsInstanceExceptions(unittest.
 
         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()
diff --git a/Objects/abstract.c b/Objects/abstract.c
--- a/Objects/abstract.c
+++ b/Objects/abstract.c
@@ -2235,7 +2235,12 @@ recursive_isinstance(PyObject *inst, PyO
 		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) &&
@@ -2273,8 +2278,12 @@ recursive_isinstance(PyObject *inst, PyO
 			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);