diff --git a/lib/Dist/Zilla/Tester.pm b/lib/Dist/Zilla/Tester.pm
index 9a25573b2..4115cfa78 100644
--- a/lib/Dist/Zilla/Tester.pm
+++ b/lib/Dist/Zilla/Tester.pm
@@ -160,7 +160,9 @@ sub minter { 'Dist::Zilla::Tester::_Minter' }
 
     local @INC = map {; ref($_) ? $_ : File::Spec->rel2abs($_) } @INC;
 
-    local $ENV{DZIL_GLOBAL_CONFIG_ROOT} = $tester_arg->{global_config_root};
+    local $ENV{DZIL_GLOBAL_CONFIG_ROOT};
+    $ENV{DZIL_GLOBAL_CONFIG_ROOT} = $tester_arg->{global_config_root}
+      if defined $ENV{DZIL_GLOBAL_CONFIG_ROOT};
 
     my $zilla = $self->$orig($arg);