--- a/testing/mozharness/mozharness/base/vcs/mercurial.py
+++ b/testing/mozharness/mozharness/base/vcs/mercurial.py
@@ -348,17 +348,17 @@ class MercurialVCS(ScriptMixin, LogMixin
         # self.vcs_config instead of passing arguments. This confuses
         # scripts that have multiple repos. This includes the clone_tools()
         # step :(
 
         if not rev and not branch:
             self.warning('did not specify revision or branch; assuming "default"')
             branch = 'default'
 
-        share_base = c.get('vcs_share_base', os.environ.get('HG_SHARE_BASE_DIR', None))
+        share_base = c.get('vcs_share_base') or os.environ.get('HG_SHARE_BASE_DIR')
         if share_base and c.get('use_vcs_unique_share'):
             # Bug 1277041 - update migration scripts to support robustcheckout
             # fake a share but don't really share
             share_base = os.path.join(share_base, hashlib.md5(dest).hexdigest())
 
         # We require shared storage is configured because it guarantees we
         # only have 1 local copy of logical repo stores.
         if not share_base: