diff --git a/xs/src/libslic3r/BoundingBox.cpp b/xs/src/libslic3r/BoundingBox.cpp
index 66beaa3d5..08be120b6 100644
--- a/xs/src/libslic3r/BoundingBox.cpp
+++ b/xs/src/libslic3r/BoundingBox.cpp
@@ -1,5 +1,6 @@
 #include "BoundingBox.hpp"
 #include <algorithm>
+#include <assert.h>
 
 namespace Slic3r {
 
@@ -125,6 +126,7 @@ template void BoundingBoxBase<Pointf>::merge(const Pointfs &points);
 template <class PointClass> void
 BoundingBoxBase<PointClass>::merge(const BoundingBoxBase<PointClass> &bb)
 {
+    assert(bb.defined || bb.min.x >= bb.max.x || bb.min.y >= bb.max.y);
     if (bb.defined) {
         if (this->defined) {
             this->min.x = std::min(bb.min.x, this->min.x);
@@ -162,6 +164,7 @@ template void BoundingBox3Base<Pointf3>::merge(const Pointf3s &points);
 template <class PointClass> void
 BoundingBox3Base<PointClass>::merge(const BoundingBox3Base<PointClass> &bb)
 {
+    assert(bb.defined || bb.min.x >= bb.max.x || bb.min.y >= bb.max.y || bb.min.z >= bb.max.z);
     if (bb.defined) {
         if (this->defined) {
             this->min.z = std::min(bb.min.z, this->min.z);
diff --git a/xs/src/libslic3r/GCode.cpp b/xs/src/libslic3r/GCode.cpp
index 478864e74..c0b0d59f7 100644
--- a/xs/src/libslic3r/GCode.cpp
+++ b/xs/src/libslic3r/GCode.cpp
@@ -710,7 +710,6 @@ bool GCode::_do_export(Print &print, FILE *file)
                     // Just wait for a bit to let the user check, that the priming succeeded.
                     fprintf(file, "M117 Verify extruder priming\nM0 S10\nM117 Printing\n");
                 }
-                write(file, this->unretract());
             } else
                 write(file, WipeTowerIntegration::prime_single_color_print(print, initial_extruder_id, *this));
         }
diff --git a/xs/src/libslic3r/PrintObject.cpp b/xs/src/libslic3r/PrintObject.cpp
index 9bf74bee7..475a320bf 100644
--- a/xs/src/libslic3r/PrintObject.cpp
+++ b/xs/src/libslic3r/PrintObject.cpp
@@ -1253,7 +1253,8 @@ void PrintObject::_slice()
                 goto end;
         delete layer;
         this->layers.pop_back();
-        this->layers.back()->upper_layer = nullptr;
+		if (! this->layers.empty())
+			this->layers.back()->upper_layer = nullptr;
     }
 end:
     ;
diff --git a/xs/src/libslic3r/TriangleMesh.cpp b/xs/src/libslic3r/TriangleMesh.cpp
index f2f05a841..38d89938a 100644
--- a/xs/src/libslic3r/TriangleMesh.cpp
+++ b/xs/src/libslic3r/TriangleMesh.cpp
@@ -574,6 +574,7 @@ BoundingBoxf3
 TriangleMesh::bounding_box() const
 {
     BoundingBoxf3 bb;
+    bb.defined = true;
     bb.min.x = this->stl.stats.min.x;
     bb.min.y = this->stl.stats.min.y;
     bb.min.z = this->stl.stats.min.z;