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
@@ -211,6 +211,7 @@ option(env='PYTHON', nargs=1, help='Python 2.7 interpr
 @imports(_from='mozbuild.virtualenv', _import='VirtualenvManager')
 @imports(_from='mozbuild.virtualenv', _import='verify_python_version')
 @imports(_from='mozbuild.virtualenv', _import='PY2')
+@imports(_from='__builtin__', _import='KeyError')
 def virtualenv_python2(env_python, build_env, mozconfig, help):
     if help:
         return
@@ -242,6 +243,12 @@ def virtualenv_python2(env_python, build_env, mozconfi
             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
 
     log.debug("python2: executable from configuration: %r" % python)
 
@@ -294,7 +301,10 @@ def virtualenv_python2(env_python, build_env, mozconfi
                 sys.executable, manager.python_path))
             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))
@@ -430,6 +440,7 @@ option(env='PYTHON3', nargs=1, help='Python 3 interpre
 @imports(_from='mozbuild.virtualenv', _import='PY3')
 @imports(_from='mozbuild.pythonutil', _import='find_python3_executable')
 @imports(_from='mozbuild.pythonutil', _import='python_executable_version')
+@imports(_from='__builtin__', _import='KeyError')
 def virtualenv_python3(env_python, build_env, mozillabuild, mozconfig, help):
     if help:
         return
@@ -465,6 +476,12 @@ def virtualenv_python3(env_python, build_env, mozillab
             python = mozconfig['vars']['added']['PYTHON3']
         elif 'PYTHON3' in mozconfig['vars']['modified']:
             python = mozconfig['vars']['modified']['PYTHON3'][1]
+        for i in ('env', 'vars'):
+            for j in ('added', 'modified'):
+                try:
+                    del mozconfig[i][j]['PYTHON3']
+                except KeyError:
+                    pass
 
     log.debug("python3: executable from configuration: %r" % python)
 
@@ -545,7 +562,10 @@ def virtualenv_python3(env_python, build_env, mozillab
                 sys.executable, manager.python_path))
             log.info('Re-executing in the virtualenv')
             if env_python:
-                del os.environ['PYTHON3']
+                try:
+                    del os.environ['PYTHON3']
+                except KeyError:
+                    pass
             # One would prefer to use os.execl, but that's completely borked on
             # Windows.
             sys.exit(subprocess.call([python] + sys.argv))