diff --git a/src/kernel/disconnected_transactions.cpp b/src/kernel/disconnected_transactions.cpp
index d8907029591..9c3f39135e1 100644
--- a/src/kernel/disconnected_transactions.cpp
+++ b/src/kernel/disconnected_transactions.cpp
@@ -51,7 +51,8 @@ size_t DisconnectedBlockTransactions::DynamicMemoryUsage() const
     iters_by_txid.reserve(iters_by_txid.size() + vtx.size());
     for (auto block_it = vtx.rbegin(); block_it != vtx.rend(); ++block_it) {
         auto it = queuedTx.insert(queuedTx.end(), *block_it);
-        iters_by_txid.emplace((*block_it)->GetHash(), it);
+        auto [_, inserted] = iters_by_txid.emplace((*block_it)->GetHash(), it);
+        assert(inserted); // callers may never pass multiple transactions with the same txid
         cachedInnerUsage += RecursiveDynamicUsage(**block_it);
     }
     return LimitMemoryUsage();