diff --git a/docs/changelog.rst b/docs/changelog.rst
index 169e4095..157ed902 100644
--- a/docs/changelog.rst
+++ b/docs/changelog.rst
@@ -139,6 +139,12 @@ Fixes
   0.2.3 behaviour of defaulting to Kqueue in this case, but still prefer
   :func:`select.poll` if it is available.
 
+* `#545 <https://github.com/dw/mitogen/issues/545>`_: an optimization
+  introduced in `#493 <https://github.com/dw/mitogen/issues/493>`_ caused a
+  64-bit integer to be assigned to a 32-bit field on ARM 32-bit targets,
+  causing runs to fail.
+
+
 Core Library
 ~~~~~~~~~~~~
 
@@ -151,6 +157,7 @@ Thanks!
 
 Mitogen would not be possible without the support of users. A huge thanks for
 bug reports, testing, features and fixes in this release contributed by
+`Fabian Arrotin <https://github.com/arrfab>`_, and
 `Petr Enkov <https://github.com/enkov>`_.
 
 
diff --git a/mitogen/core.py b/mitogen/core.py
index dcca2e91..25732896 100644
--- a/mitogen/core.py
+++ b/mitogen/core.py
@@ -2140,7 +2140,7 @@ class Latch(object):
             return rsock, wsock
 
     COOKIE_MAGIC, = struct.unpack('L', b('LTCH') * (struct.calcsize('L')//4))
-    COOKIE_FMT = 'Llll'
+    COOKIE_FMT = '>Qqqq'  # #545: id() and get_ident() may exceed long on armhfp.
     COOKIE_SIZE = struct.calcsize(COOKIE_FMT)
 
     def _make_cookie(self):