diff --git a/src/txgraph.cpp b/src/txgraph.cpp
index fa1395273da..22dce81fee1 100644
--- a/src/txgraph.cpp
+++ b/src/txgraph.cpp
@@ -1880,7 +1880,7 @@ TxGraph::GraphIndex TxGraphImpl::CountDistinctClusters(std::span<const Ref* cons
     std::vector<Cluster*> clusters;
     clusters.reserve(refs.size());
     for (const Ref* ref : refs) {
-        if (ref == nullptr) continue;
+        Assume(ref);
         if (GetRefGraph(*ref) == nullptr) continue;
         Assume(GetRefGraph(*ref) == this);
         auto cluster = FindCluster(GetRefIndex(*ref), level);
diff --git a/src/txgraph.h b/src/txgraph.h
index 803696440c2..91c3bd5cb17 100644
--- a/src/txgraph.h
+++ b/src/txgraph.h
@@ -159,8 +159,8 @@ public:
     virtual std::strong_ordering CompareMainOrder(const Ref& a, const Ref& b) noexcept = 0;
     /** Count the number of distinct clusters that the specified transactions belong to. If
      *  main_only is false and a staging graph exists, staging clusters are counted. Otherwise,
-     *  main clusters are counted. Refs that do not exist in the queried graph are ignored. The
-     *  queried graph must not be oversized. */
+     *  main clusters are counted. Refs that do not exist in the queried graph are ignored. Refs
+     *  can not be null. The queried graph must not be oversized. */
     virtual GraphIndex CountDistinctClusters(std::span<const Ref* const>, bool main_only = false) noexcept = 0;
 
     /** Perform an internal consistency check on this object. */