diff --git a/lib/ExtUtils/ParseXS.pm b/lib/ExtUtils/ParseXS.pm
index f67a82d..c17372a 100644
--- a/lib/ExtUtils/ParseXS.pm
+++ b/lib/ExtUtils/ParseXS.pm
@@ -686,7 +686,7 @@ EOF
         var         => $_,
         do_setmagic => $self->{DoSetMagic},
         do_push     => undef,
-      } ) for grep $self->{in_out}->{$_} =~ /OUT$/, keys %{ $self->{in_out} };
+      } ) for grep $self->{in_out}->{$_} =~ /OUT$/, sort keys %{ $self->{in_out} };
 
       my $prepush_done;
       # all OUTPUT done, so now push the return value on the stack