diff --git a/source/blender/geometry/intern/mesh_triangulate.cc b/source/blender/geometry/intern/mesh_triangulate.cc
index 2c977e416f3..5ce02e25a69 100644
--- a/source/blender/geometry/intern/mesh_triangulate.cc
+++ b/source/blender/geometry/intern/mesh_triangulate.cc
@@ -554,12 +554,13 @@ static IndexMask calc_unselected_faces(const Mesh &mesh,
       memory,
       [&](const IndexMaskSegment universe_segment, IndexRangesBuilder<int16_t> &builder) {
         if (unique_sorted_indices::non_empty_is_range(universe_segment.base_span())) {
-          const IndexRange segment_range(universe_segment[0], universe_segment.size());
+          const IndexRange universe_as_range = unique_sorted_indices::non_empty_as_range(
+              universe_segment.base_span());
+          const IndexRange segment_range = universe_as_range.shift(universe_segment.offset());
           const OffsetIndices segment_faces = src_faces.slice(segment_range);
           if (segment_faces.total_size() == segment_faces.size() * 3) {
             /* All faces in segment are triangles. */
-            builder.add_range(universe_segment.base_span().first(),
-                              universe_segment.base_span().last());
+            builder.add_range(universe_as_range.start(), universe_as_range.one_after_last());
             return universe_segment.offset();
           }
         }