diff --git a/lib/Slic3r/GCode.pm b/lib/Slic3r/GCode.pm
index cea5dcd54d..573d14bcb5 100644
--- a/lib/Slic3r/GCode.pm
+++ b/lib/Slic3r/GCode.pm
@@ -68,7 +68,7 @@ sub set_origin {
 
 sub init_external_mp {
     my ($self, $islands) = @_;
-    $self->external_mp(Slic3r::MotionPlanner->new($islands));
+    $self->_external_mp(Slic3r::MotionPlanner->new($islands));
 }
 
 sub change_layer {