--- a/testing/mozharness/mozharness/base/script.py
+++ b/testing/mozharness/mozharness/base/script.py
@@ -117,17 +117,19 @@ class PlatformMixin(object):
         if sys.platform.startswith("linux"):
             return True
 
     def _is_64_bit(self):
         if self._is_darwin():
             # osx is a special snowflake and to ensure the arch, it is better to use the following
             return sys.maxsize > 2**32  # context: https://docs.python.org/2/library/platform.html
         else:
-            return '64' in platform.architecture()[0]  # architecture() returns (bits, linkage)
+            # Using machine() gives you the architecture of the host rather
+            # than the build type of the Python binary
+            return '64' in platform.machine()
 
 
 # ScriptMixin {{{1
 class ScriptMixin(PlatformMixin):
     """This mixin contains simple filesystem commands and the like.
 
     It also contains some very special but very complex methods that,
     together with logging and config, provide the base for all scripts