diff --git a/src/libslic3r/PrintObject.cpp b/src/libslic3r/PrintObject.cpp
index f344d79b9..a409f12e1 100644
--- a/src/libslic3r/PrintObject.cpp
+++ b/src/libslic3r/PrintObject.cpp
@@ -2014,7 +2014,7 @@ std::vector<ExPolygons> PrintObject::slice_region(size_t region_id, const std::v
 	return this->slice_volumes(z, mode, volumes);
 }
 
-// Z ranges are not applicable to modifier meshes, therefore a sinle volume will be found in volume_and_range at most once.
+// Z ranges are not applicable to modifier meshes, therefore a single volume will be found in volume_and_range at most once.
 std::vector<ExPolygons> PrintObject::slice_modifiers(size_t region_id, const std::vector<float> &slice_zs) const
 {
 	std::vector<ExPolygons> out;
@@ -2080,10 +2080,12 @@ std::vector<ExPolygons> PrintObject::slice_modifiers(size_t region_id, const std
 								ranges.emplace_back(volumes_and_ranges[j].first);
 			                // slicing in parallel
 			                std::vector<ExPolygons> this_slices = this->slice_volume(slice_zs, ranges, SlicingMode::Regular, *model_volume);
+                            // Variable this_slices could be empty if no value of slice_zs is within any of the ranges of this volume.
 			                if (out.empty()) {
 			                	out = std::move(this_slices);
 			                	merge.assign(out.size(), false);
-			                } else {
+			                } else if (!this_slices.empty()) {
+                                assert(out.size() == this_slices.size());
 			                	for (size_t i = 0; i < out.size(); ++ i)
                                     if (! this_slices[i].empty()) {
 			                			if (! out[i].empty()) {
@@ -2188,7 +2190,7 @@ std::vector<ExPolygons> PrintObject::slice_volume(const std::vector<float> &z, S
     return layers;
 }
 
-// Filter the zs not inside the ranges. The ranges are closed at the botton and open at the top, they are sorted lexicographically and non overlapping.
+// Filter the zs not inside the ranges. The ranges are closed at the bottom and open at the top, they are sorted lexicographically and non overlapping.
 std::vector<ExPolygons> PrintObject::slice_volume(const std::vector<float> &z, const std::vector<t_layer_height_range> &ranges, SlicingMode mode, const ModelVolume &volume) const
 {
 	std::vector<ExPolygons> out;