diff --git a/source/blender/editors/sculpt_paint/grease_pencil_trace.cc b/source/blender/editors/sculpt_paint/grease_pencil_trace.cc
index 2a4ae4ea6f3..5000e4e2d25 100644
--- a/source/blender/editors/sculpt_paint/grease_pencil_trace.cc
+++ b/source/blender/editors/sculpt_paint/grease_pencil_trace.cc
@@ -98,6 +98,8 @@ struct TraceJob {
   TraceMode mode;
   /* Custom source frame, allows overriding the default scene frame. */
   int frame_number;
+  int foreground_material_index;
+  int background_material_index;
 
   bool success;
   bool was_canceled;
@@ -207,17 +209,18 @@ static bke::CurvesGeometry grease_pencil_trace_image(TraceJob &trace_job, const
 
   /* Assign different materials to foreground curves and hole curves. */
   bke::MutableAttributeAccessor attributes = trace_curves.attributes_for_write();
-  const int material_fg = ensure_foreground_material(
-      trace_job.bmain, trace_job.ob_grease_pencil, "Stroke");
-  const int material_bg = ensure_background_material(
-      trace_job.bmain, trace_job.ob_grease_pencil, "Holdout");
+  BLI_assert_msg(trace_job.foreground_material_index >= 0,
+                 "ensure_foreground_material must be called on the main thread");
+  BLI_assert_msg(trace_job.background_material_index >= 0,
+                 "ensure_background_material must be called on the main thread");
   const VArraySpan<bool> holes = *attributes.lookup<bool>(hole_attribute_id);
   bke::SpanAttributeWriter<int> material_indices = attributes.lookup_or_add_for_write_span<int>(
       "material_index", bke::AttrDomain::Curve);
   threading::parallel_for(trace_curves.curves_range(), 4096, [&](const IndexRange range) {
     for (const int curve_i : range) {
       const bool is_hole = holes[curve_i];
-      material_indices.span[curve_i] = (is_hole ? material_bg : material_fg);
+      material_indices.span[curve_i] = (is_hole ? trace_job.background_material_index :
+                                                  trace_job.foreground_material_index);
     }
   });
   material_indices.finish();
@@ -424,6 +427,12 @@ static wmOperatorStatus grease_pencil_trace_image_exec(bContext *C, wmOperator *
   /* Back to active base. */
   blender::ed::object::base_activate(job->C, job->base_active);
 
+  /* Create materials on the main thread before starting the job. */
+  job->foreground_material_index = ensure_foreground_material(
+      job->bmain, job->ob_grease_pencil, "Stroke");
+  job->background_material_index = ensure_background_material(
+      job->bmain, job->ob_grease_pencil, "Holdout");
+
   if ((job->image->source == IMA_SRC_FILE) || (job->frame_number > 0)) {
     wmJobWorkerStatus worker_status = {};
     trace_start_job(job, &worker_status);