diff --git a/xs/src/libslic3r/Flow.cpp b/xs/src/libslic3r/Flow.cpp
index 1f787f50e..25a44ea8f 100644
--- a/xs/src/libslic3r/Flow.cpp
+++ b/xs/src/libslic3r/Flow.cpp
@@ -131,9 +131,13 @@ float Flow::spacing(const Flow &other) const
 // This method returns extrusion volume per head move unit.
 double Flow::mm3_per_mm() const 
 {
-    return this->bridge ?
+    double res = this->bridge ?
+        // Area of a circle with dmr of this->width.
         (this->width * this->width) * 0.25 * PI :
-        this->width * this->height + 0.25 * (this->height * this->height) / (PI - 4.0);
+        // Rectangle with semicircles at the ends. ~ h (w - 0.215 h)
+        this->height * (this->width - this->height * (1. - 0.25 * PI));
+    assert(res > 0.);
+    return res;
 }
 
 Flow support_material_flow(const PrintObject *object, float layer_height)
diff --git a/xs/src/libslic3r/GCodeWriter.cpp b/xs/src/libslic3r/GCodeWriter.cpp
index 40500d084..abf55114b 100644
--- a/xs/src/libslic3r/GCodeWriter.cpp
+++ b/xs/src/libslic3r/GCodeWriter.cpp
@@ -3,6 +3,7 @@
 #include <iomanip>
 #include <iostream>
 #include <map>
+#include <assert.h>
 
 #define FLAVOR_IS(val) this->config.gcode_flavor == val
 #define FLAVOR_IS_NOT(val) this->config.gcode_flavor != val
@@ -259,6 +260,8 @@ std::string GCodeWriter::toolchange(unsigned int extruder_id)
 
 std::string GCodeWriter::set_speed(double F, const std::string &comment, const std::string &cooling_marker) const
 {
+    assert(F > 0.);
+    assert(F < 100000.);
     std::ostringstream gcode;
     gcode << "G1 F" << F;
     COMMENT(comment);
diff --git a/xs/src/slic3r/GUI/3DScene.hpp b/xs/src/slic3r/GUI/3DScene.hpp
index 850b72000..27eeb7ca8 100644
--- a/xs/src/slic3r/GUI/3DScene.hpp
+++ b/xs/src/slic3r/GUI/3DScene.hpp
@@ -158,6 +158,7 @@ public:
     BoundingBoxf3 bounding_box() const {
         BoundingBoxf3 bbox;
         if (! this->vertices_and_normals_interleaved.empty()) {
+            bbox.defined = true;
             bbox.min.x = bbox.max.x = this->vertices_and_normals_interleaved[3];
             bbox.min.y = bbox.max.y = this->vertices_and_normals_interleaved[4];
             bbox.min.z = bbox.max.z = this->vertices_and_normals_interleaved[5];