diff --git a/src/txgraph.cpp b/src/txgraph.cpp
index 22dce81fee1..9e4bf116b02 100644
--- a/src/txgraph.cpp
+++ b/src/txgraph.cpp
@@ -1595,6 +1595,7 @@ std::vector<TxGraph::Ref*> TxGraphImpl::GetAncestorsUnion(std::span<const Ref* c
     std::vector<std::pair<Cluster*, DepGraphIndex>> matches;
     matches.reserve(args.size());
     for (auto arg : args) {
+        Assume(arg);
         // Skip empty Refs.
         if (GetRefGraph(*arg) == nullptr) continue;
         Assume(GetRefGraph(*arg) == this);
@@ -1627,6 +1628,7 @@ std::vector<TxGraph::Ref*> TxGraphImpl::GetDescendantsUnion(std::span<const Ref*
     std::vector<std::pair<Cluster*, DepGraphIndex>> matches;
     matches.reserve(args.size());
     for (auto arg : args) {
+        Assume(arg);
         // Skip empty Refs.
         if (GetRefGraph(*arg) == nullptr) continue;
         Assume(GetRefGraph(*arg) == this);
diff --git a/src/txgraph.h b/src/txgraph.h
index 91c3bd5cb17..cdfb2fe6dec 100644
--- a/src/txgraph.h
+++ b/src/txgraph.h
@@ -144,11 +144,11 @@ public:
     virtual std::vector<Ref*> GetDescendants(const Ref& arg, bool main_only = false) noexcept = 0;
     /** Like GetAncestors, but return the Refs for all transactions in the union of the provided
      *  arguments' ancestors (each transaction is only reported once). Refs that do not exist in
-     *  the queried graph are ignored. */
+     *  the queried graph are ignored. Null refs are not allowed. */
     virtual std::vector<Ref*> GetAncestorsUnion(std::span<const Ref* const> args, bool main_only = false) noexcept = 0;
     /** Like GetDescendants, but return the Refs for all transactions in the union of the provided
      *  arguments' descendants (each transaction is only reported once). Refs that do not exist in
-     *  the queried graph are ignored. */
+     *  the queried graph are ignored. Null refs are not allowed. */
     virtual std::vector<Ref*> GetDescendantsUnion(std::span<const Ref* const> args, bool main_only = false) noexcept = 0;
     /** Get the total number of transactions in the graph. If main_only is false and a staging
      *  graph exists, it is queried; otherwise the main graph is queried. This is available even