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
@@ -251,6 +251,7 @@ option(env='PYTHON3', nargs=1, help='Python 3 interpre
 @imports(_from='mozbuild.pythonutil', _import='find_python3_executable')
 @imports(_from='mozbuild.pythonutil', _import='python_executable_version')
 @imports(_from='six', _import='ensure_text')
+@imports(_from='__builtin__', _import='KeyError')
 def virtualenv_python3(env_python, virtualenv_name, build_env, mozconfig, help):
     # Avoid re-executing python when running configure --help.
     if help:
@@ -283,6 +284,12 @@ def virtualenv_python3(env_python, build_env, mozconfi
             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)
 
@@ -365,7 +372,10 @@ def virtualenv_python3(env_python, build_env, mozconfi
             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
         # Homebrew on macOS will change Python's sys.executable to a custom
         # value which messes with mach's virtualenv handling code. Override
         # Homebrew's changes with the correct sys.executable value.