diff --git a/image_analysis/beam_stop/ShadowFinder.cpp b/image_analysis/beam_stop/ShadowFinder.cpp
index 29a4f83c3..715ad1198 100644
--- a/image_analysis/beam_stop/ShadowFinder.cpp
+++ b/image_analysis/beam_stop/ShadowFinder.cpp
@@ -958,7 +958,7 @@ std::vector<uint32_t> ShadowFinder::GetMask(size_t nthreads) const {
         }
     });
 
-    std::vector<char> dim(n_pixels, 0);
+    Plane<char> dim = filled_plane<char>(n_pixels, 0, nthreads);
     ParallelChunks(n_pixels, nthreads, [&](int lo, int hi) {
         for (int i = lo; i < hi; i++) {
             if (!valid[i] || region[i] || ratio[i] >= PENUMBRA_RATIO)