diff --git a/source/blender/draw/engines/gpencil/gpencil_draw_data.cc b/source/blender/draw/engines/gpencil/gpencil_draw_data.cc
index 97e80c65bdb..ae3798df3eb 100644
--- a/source/blender/draw/engines/gpencil/gpencil_draw_data.cc
+++ b/source/blender/draw/engines/gpencil/gpencil_draw_data.cc
@@ -304,6 +304,7 @@ void gpencil_material_resources_get(GPENCIL_MaterialPool *first_pool,
                                     GPUUniformBuf **r_ubo_mat)
 {
   GPENCIL_MaterialPool *matpool = first_pool;
+  BLI_assert(mat_id >= 0);
   int pool_id = mat_id / GPENCIL_MATERIAL_BUFFER_LEN;
   for (int i = 0; i < pool_id; i++) {
     matpool = matpool->next;
diff --git a/source/blender/draw/engines/gpencil/gpencil_engine_c.cc b/source/blender/draw/engines/gpencil/gpencil_engine_c.cc
index c34fa5346c6..84f79e5a544 100644
--- a/source/blender/draw/engines/gpencil/gpencil_engine_c.cc
+++ b/source/blender/draw/engines/gpencil/gpencil_engine_c.cc
@@ -790,7 +790,10 @@ static GPENCIL_tObject *grease_pencil_object_cache_populate(GPENCIL_PrivateData
 
     visible_strokes.foreach_index([&](const int stroke_i, const int pos) {
       const IndexRange points = points_by_curve[stroke_i];
-      const int material_index = stroke_materials[stroke_i];
+      /* The material index is allowed to be negative as it's stored as a generic attribure. We
+       * clamp it here to avoid crashing in the rendering code. Any stroke with a material < 0 will
+       * use the first material in the first material slot.*/
+      const int material_index = std::max(stroke_materials[stroke_i], 0);
       const MaterialGPencilStyle *gp_style = BKE_gpencil_material_settings(ob, material_index + 1);
 
       const bool hide_material = (gp_style->flag & GP_MATERIAL_HIDE) != 0;
diff --git a/source/blender/editors/grease_pencil/intern/grease_pencil_geom.cc b/source/blender/editors/grease_pencil/intern/grease_pencil_geom.cc
index 3063cb58eb3..2cb9c669bfc 100644
--- a/source/blender/editors/grease_pencil/intern/grease_pencil_geom.cc
+++ b/source/blender/editors/grease_pencil/intern/grease_pencil_geom.cc
@@ -697,7 +697,7 @@ bke::CurvesGeometry create_curves_outline(const bke::greasepencil::Drawing &draw
   const VArray<int8_t> src_end_caps = *src_attributes.lookup_or_default<int8_t>(
       "end_cap", bke::AttrDomain::Curve, GP_STROKE_CAP_ROUND);
   const VArray<int> src_material_index = *src_attributes.lookup_or_default(
-      "material_index", bke::AttrDomain::Curve, -1);
+      "material_index", bke::AttrDomain::Curve, 0);
 
   /* Transform positions. */
   Array<float3> transformed_positions(src_positions.size());
diff --git a/source/blender/editors/grease_pencil/intern/grease_pencil_utils.cc b/source/blender/editors/grease_pencil/intern/grease_pencil_utils.cc
index 0f140c4f6c7..48a62129174 100644
--- a/source/blender/editors/grease_pencil/intern/grease_pencil_utils.cc
+++ b/source/blender/editors/grease_pencil/intern/grease_pencil_utils.cc
@@ -888,7 +888,7 @@ IndexMask retrieve_visible_strokes(Object &object,
 
   /* Get all the strokes that have their material visible. */
   const VArray<int> materials = *attributes.lookup_or_default<int>(
-      "material_index", bke::AttrDomain::Curve, -1);
+      "material_index", bke::AttrDomain::Curve, 0);
   return IndexMask::from_predicate(
       curves_range, GrainSize(4096), memory, [&](const int64_t curve_i) {
         const int material_index = materials[curve_i];