diff --git a/Lib/contextlib.py b/Lib/contextlib.py
index 282fc510045..d329087c974 100644
--- a/Lib/contextlib.py
+++ b/Lib/contextlib.py
@@ -34,6 +34,13 @@ class GeneratorContextManager(object):
             except StopIteration:
                 return True
             except:
+                # only re-raise if it's *not* the exception that was
+                # passed to throw(), because __exit__() must not raise
+                # an exception unless __exit__() itself failed.  But throw()
+                # has to raise the exception to signal propagation, so this
+                # fixes the impedance mismatch between the throw() protocol
+                # and the __exit__() protocol.
+                #
                 if sys.exc_info()[1] is not value:
                     raise