diff --git a/notmuch.c b/notmuch.c
index 68a785e2..c75b0188 100644
--- a/notmuch.c
+++ b/notmuch.c
@@ -429,11 +429,18 @@ notmuch_command (notmuch_database_t *notmuch,
  * false on errors.
  */
 static bool
-try_external_command (char *argv[])
+try_external_command (const char *config_file_name, char *argv[])
 {
     char *old_argv0 = argv[0];
     bool ret = true;
 
+    if (config_file_name) {
+	if (setenv ("NOTMUCH_CONFIG", config_file_name, 1)) {
+	    perror ("setenv");
+	    exit (1);
+	}
+    }
+
     argv[0] = talloc_asprintf (NULL, "notmuch-%s", old_argv0);
 
     /*
@@ -493,7 +500,7 @@ main (int argc, char *argv[])
     /* if command->function is NULL, try external command */
     if (! command || ! command->function) {
 	/* This won't return if the external command is found. */
-	if (try_external_command (argv + opt_index))
+	if (try_external_command (config_file_name, argv + opt_index))
 	    fprintf (stderr, "Error: Unknown command '%s' (see \"notmuch help\")\n",
 		     command_name);
 	ret = EXIT_FAILURE;
diff --git a/test/T405-external.sh b/test/T405-external.sh
index b121fc72..0e1d9646 100755
--- a/test/T405-external.sh
+++ b/test/T405-external.sh
@@ -44,7 +44,6 @@ EOF
 test_expect_equal_file_nonempty EXPECTED OUTPUT
 
 test_begin_subtest "NOTMUCH_CONFIG is set by --config"
-test_subtest_known_broken
 cp "${NOTMUCH_CONFIG}" "${NOTMUCH_CONFIG}.alternate"
 cat <<EOF > EXPECTED
 ${NOTMUCH_CONFIG}.alternate