--- a/tools/docs/mach_commands.py
+++ b/tools/docs/mach_commands.py
@@ -42,31 +42,32 @@ class Documentation(MachCommandBase):
 
         import sphinx
         import webbrowser
 
         if not outdir:
             outdir = os.path.join(self.topobjdir, 'docs')
         if not what:
             what = [os.path.join(self.topsrcdir, 'tools')]
-        outdir = os.path.join(outdir, format)
+
+        format_outdir = os.path.join(outdir, format)
 
         generated = []
         failed = []
         for path in what:
             path = os.path.normpath(os.path.abspath(path))
             docdir = self._find_doc_dir(path)
 
             if not docdir:
                 failed.append((path, 'could not find docs at this location'))
                 continue
 
             # find project name to use as a namespace within `outdir`
             project = self._find_project_name(docdir)
-            savedir = os.path.join(outdir, project)
+            savedir = os.path.join(format_outdir, project)
 
             args = [
                 'sphinx',
                 '-b', format,
                 docdir,
                 savedir,
             ]
             result = sphinx.build_main(args)
@@ -87,17 +88,18 @@ class Documentation(MachCommandBase):
             return die('failed to generate documentation:\n%s' % '\n'.join(failed))
 
         if http is not None:
             host, port = http.split(':', 1)
             addr = (host, int(port))
             if len(addr) != 2:
                 return die('invalid address: %s' % http)
 
-            httpd = mozhttpd.MozHttpd(host=addr[0], port=addr[1], docroot=outdir)
+            httpd = mozhttpd.MozHttpd(host=addr[0], port=addr[1],
+                                      docroot=format_outdir)
             print('listening on %s:%d' % addr)
             httpd.start(block=True)
 
     def _find_project_name(self, path):
         import imp
         path = os.path.join(path, 'conf.py')
         with open(path, 'r') as fh:
             conf = imp.load_module('doc_conf', fh, path,