diff --git a/src/search.cpp b/src/search.cpp
index accf8be3516..9094224c6ff 100644
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -775,6 +775,7 @@ Value Search::Worker::search(
     // Step 7. Razoring (~1 Elo)
     // If eval is really low, check with qsearch if we can exceed alpha. If the
     // search suggests we cannot exceed alpha, return a speculative fail low.
+    // For PvNodes, we must have a guard against mates being returned.
     if (!PvNode && eval < alpha - 469 - 307 * depth * depth)
         return qsearch<NonPV>(pos, ss, alpha - 1, alpha);