diff --git a/src/libslic3r/SLAPrint.hpp b/src/libslic3r/SLAPrint.hpp
index 668def6a8..94040ce38 100644
--- a/src/libslic3r/SLAPrint.hpp
+++ b/src/libslic3r/SLAPrint.hpp
@@ -6,13 +6,13 @@
 
 namespace Slic3r {
 
-enum SLAPrintStep {
+enum SLAPrintStep : unsigned int {
 	slapsRasterize,
 	slapsValidate,
 	slapsCount
 };
 
-enum SLAPrintObjectStep {
+enum SLAPrintObjectStep : unsigned int {
 	slaposObjectSlice,
 	slaposSupportIslands,
 	slaposSupportPoints,
diff --git a/src/slic3r/GUI/3DScene.hpp b/src/slic3r/GUI/3DScene.hpp
index e611e7f54..9761ba3b6 100644
--- a/src/slic3r/GUI/3DScene.hpp
+++ b/src/slic3r/GUI/3DScene.hpp
@@ -18,7 +18,7 @@ class Print;
 class PrintObject;
 class SLAPrint;
 class SLAPrintObject;
-enum  SLAPrintObjectStep;
+enum  SLAPrintObjectStep : unsigned int;
 class Model;
 class ModelObject;
 class GCodePreviewData;
diff --git a/src/slic3r/GUI/GLCanvas3D.cpp b/src/slic3r/GUI/GLCanvas3D.cpp
index f9daa36d0..6d089b5b2 100644
--- a/src/slic3r/GUI/GLCanvas3D.cpp
+++ b/src/slic3r/GUI/GLCanvas3D.cpp
@@ -1220,7 +1220,7 @@ void GLCanvas3D::Selection::add(unsigned int volume_idx, bool as_single_selectio
     {
     case Volume:
     {
-        if (is_empty() || (volume->instance_idx() == get_instance_idx()))
+        if (volume->volume_idx() >= 0 && (is_empty() || (volume->instance_idx() == get_instance_idx())))
             _add_volume(volume_idx);
 
         break;