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
@@ -238,6 +238,7 @@ option(
 @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:
@@ -271,6 +272,12 @@ def virtualenv_python3(env_python, virtualenv_name, bu
             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)
 
@@ -364,7 +371,10 @@ def virtualenv_python3(env_python, virtualenv_name, bu
         )
         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.