diff --git a/build/moz.configure/init.configure b/build/moz.configure/init.configure
index 855214a..1e91d51 100644
--- build/moz.configure/init.configure
+++ build/moz.configure/init.configure
@@ -165,16 +165,17 @@ option(env='PYTHON', nargs=1, help='Python interpreter')
 # ==============================================================
 @depends('PYTHON', check_build_environment, mozconfig, '--help')
 @imports('os')
 @imports('sys')
 @imports('subprocess')
 @imports(_from='mozbuild.configure.util', _import='LineIO')
 @imports(_from='mozbuild.virtualenv', _import='VirtualenvManager')
 @imports(_from='mozbuild.virtualenv', _import='verify_python_version')
+@imports(_from='__builtin__', _import='KeyError')
 @imports('distutils.sysconfig')
 def virtualenv_python(env_python, build_env, mozconfig, help):
     if help:
         return
 
     python = env_python[0] if env_python else None
 
     # Ideally we'd rely on the mozconfig injection from mozconfig_options,
@@ -184,16 +185,22 @@ def virtualenv_python(env_python, build_env, mozconfig, help):
         if 'PYTHON' in mozconfig['env']['added']:
             python = mozconfig['env']['added']['PYTHON']
         elif 'PYTHON' in mozconfig['env']['modified']:
             python = mozconfig['env']['modified']['PYTHON'][1]
         elif 'PYTHON' in mozconfig['vars']['added']:
             python = mozconfig['vars']['added']['PYTHON']
         elif 'PYTHON' in mozconfig['vars']['modified']:
             python = mozconfig['vars']['modified']['PYTHON'][1]
+        for i in ('env', 'vars'):
+            for j in ('added', 'modified'):
+                try:
+                    del mozconfig[i][j]['PYTHON']
+                except KeyError:
+                    pass
 
     with LineIO(lambda l: log.error(l)) as out:
         verify_python_version(out)
     topsrcdir, topobjdir = build_env.topsrcdir, build_env.topobjdir
     if topobjdir.endswith('/js/src'):
         topobjdir = topobjdir[:-7]
 
     with LineIO(lambda l: log.info(l)) as out:
@@ -255,17 +262,20 @@ def virtualenv_python(env_python, build_env, mozconfig
         log.info('Creating Python environment')
         manager.build(python)
 
     python = normsep(manager.python_path)
 
     if not normsep(sys.executable).startswith(normsep(virtualenvs_root)):
         log.info('Reexecuting in the virtualenv')
         if env_python:
-            del os.environ['PYTHON']
+            try:
+                del os.environ['PYTHON']
+            except KeyError:
+                pass
         # One would prefer to use os.execl, but that's completely borked on
         # Windows.
         sys.exit(subprocess.call([python] + sys.argv))
 
     # We are now in the virtualenv
     if not distutils.sysconfig.get_python_lib():
         die('Could not determine python site packages directory')