diff --git a/src/coinjoin/coinjoin.cpp b/src/coinjoin/coinjoin.cpp
index 9c5d99ea3f81..33fbc43d6b0e 100644
--- a/src/coinjoin/coinjoin.cpp
+++ b/src/coinjoin/coinjoin.cpp
@@ -301,21 +301,17 @@ bool CoinJoin::IsCollateralValid(ChainstateManager& chainman, const llmq::CInsta
         }
     }
 
+    LOCK(::cs_main);
+    CCoinsViewMemPool viewMemPool(&chainman.ActiveChainstate().CoinsTip(), mempool);
+
     for (const auto& txin : txCollateral.vin) {
         Coin coin;
-        auto mempoolTx = mempool.get(txin.prevout.hash);
-        if (mempoolTx != nullptr) {
-            if (mempool.isSpent(txin.prevout) || !isman.IsLocked(txin.prevout.hash)) {
-                LogPrint(BCLog::COINJOIN, "CoinJoin::IsCollateralValid -- spent or non-locked mempool input! txin=%s\n", txin.ToString());
-                return false;
-            }
-            nValueIn += mempoolTx->vout[txin.prevout.n].nValue;
-        } else if (GetUTXOCoin(chainman.ActiveChainstate(), txin.prevout, coin)) {
-            nValueIn += coin.out.nValue;
-        } else {
-            LogPrint(BCLog::COINJOIN, "CoinJoin::IsCollateralValid -- Unknown inputs in collateral transaction, txCollateral=%s", txCollateral.ToString()); /* Continued */
+        if (!viewMemPool.GetCoin(txin.prevout, coin) || coin.IsSpent() ||
+            (coin.nHeight == MEMPOOL_HEIGHT && !isman.IsLocked(txin.prevout.hash))) {
+            LogPrint(BCLog::COINJOIN, "CoinJoin::IsCollateralValid -- missing, spent or non-locked mempool input! txin=%s\n", txin.ToString());
             return false;
         }
+        nValueIn += coin.out.nValue;
     }
 
     //collateral transactions are required to pay out a small fee to the miners
@@ -326,12 +322,9 @@ bool CoinJoin::IsCollateralValid(ChainstateManager& chainman, const llmq::CInsta
 
     LogPrint(BCLog::COINJOIN, "CoinJoin::IsCollateralValid -- %s", txCollateral.ToString()); /* Continued */
 
-    {
-        LOCK(::cs_main);
-        if (!ATMPIfSaneFee(chainman, MakeTransactionRef(txCollateral), /*test_accept=*/true)) {
-            LogPrint(BCLog::COINJOIN, "CoinJoin::IsCollateralValid -- didn't pass ATMPIfSaneFee()\n");
-            return false;
-        }
+    if (!ATMPIfSaneFee(chainman, MakeTransactionRef(txCollateral), /*test_accept=*/true)) {
+        LogPrint(BCLog::COINJOIN, "CoinJoin::IsCollateralValid -- didn't pass ATMPIfSaneFee()\n");
+        return false;
     }
 
     return true;
diff --git a/src/llmq/dkgsession.cpp b/src/llmq/dkgsession.cpp
index fba7e8b38d39..db1cb05311c4 100644
--- a/src/llmq/dkgsession.cpp
+++ b/src/llmq/dkgsession.cpp
@@ -410,21 +410,20 @@ bool CDKGSession::PreVerifyMessage(const CDKGJustification& qj, bool& retBan) co
     }
 
     std::set<size_t> contributionsSet;
-    for (const auto& p : qj.contributions) {
-        if (p.index > members.size()) {
+    for (const auto& [index, skContribution] : qj.contributions) {
+        if (GetMemberAtIndex(index) == nullptr) {
             logger.Batch("invalid contribution index");
             retBan = true;
             return false;
         }
 
-        if (!contributionsSet.emplace(p.index).second) {
+        if (!contributionsSet.emplace(index).second) {
             logger.Batch("duplicate contribution index");
             retBan = true;
             return false;
         }
 
-        const auto& skShare = p.key;
-        if (!skShare.IsValid()) {
+        if (!skContribution.IsValid()) {
             logger.Batch("invalid contribution");
             retBan = true;
             return false;
@@ -482,8 +481,9 @@ std::optional<CInv> CDKGSession::ReceiveMessage(const CDKGJustification& qj)
         return inv;
     }
 
-    for (const auto& p : qj.contributions) {
-        const auto& member2 = members[p.index];
+    for (const auto& [index, skContribution] : qj.contributions) {
+        const auto* member2 = GetMemberAtIndex(index);
+        assert(member2);
 
         if (member->complaintsFromOthers.count(member2->dmn->proTxHash) == 0) {
             logger.Batch("got justification from %s for %s even though he didn't complain",
@@ -499,14 +499,16 @@ std::optional<CInv> CDKGSession::ReceiveMessage(const CDKGJustification& qj)
 
     std::list<std::future<bool>> futures;
     for (const auto& [index, skContribution] : qj.contributions) {
-        const auto& member2 = members[index];
+        const auto* member2 = GetMemberAtIndex(index);
+        assert(member2);
 
         // watch out to not bail out before these async calls finish (they rely on valid references)
         futures.emplace_back(blsWorker.AsyncVerifyContributionShare(member2->id, receivedVvecs[member->idx], skContribution));
     }
     auto resultIt = futures.begin();
     for (const auto& [index, skContribution] : qj.contributions) {
-        const auto& member2 = members[index];
+        const auto* member2 = GetMemberAtIndex(index);
+        assert(member2);
 
         bool result = (resultIt++)->get();
         if (!result) {
@@ -683,6 +685,12 @@ CDKGMember* CDKGSession::GetMember(const uint256& proTxHash) const
     return members[it->second].get();
 }
 
+CDKGMember* CDKGSession::GetMemberAtIndex(size_t index) const
+{
+    if (index >= members.size()) return nullptr;
+    return members[index].get();
+}
+
 void CDKGSession::MarkBadMember(size_t idx)
 {
     auto* member = members.at(idx).get();
diff --git a/src/llmq/dkgsession.h b/src/llmq/dkgsession.h
index f9a4ce756e92..95529b2bdde6 100644
--- a/src/llmq/dkgsession.h
+++ b/src/llmq/dkgsession.h
@@ -385,6 +385,7 @@ class CDKGSession
 public:
     [[nodiscard]] bool AreWeMember() const { return !myProTxHash.IsNull(); }
     [[nodiscard]] CDKGMember* GetMember(const uint256& proTxHash) const;
+    [[nodiscard]] CDKGMember* GetMemberAtIndex(size_t index) const;
     [[nodiscard]] std::optional<size_t> GetMyMemberIndex() const { return myIdx; }
     [[nodiscard]] const Uint256HashSet& RelayMembers() const { return relayMembers; }
     [[nodiscard]] const CBlockIndex* BlockIndex() const { return m_quorum_base_block_index; }
diff --git a/src/serialize.h b/src/serialize.h
index 027f9d87c434..2e119d340081 100644
--- a/src/serialize.h
+++ b/src/serialize.h
@@ -18,6 +18,7 @@
 #include <list>
 #include <map>
 #include <memory>
+#include <optional>
 #include <set>
 #include <string>
 #include <string.h>
@@ -459,10 +460,10 @@ void ReadFixedBitSet(Stream& s, std::vector<bool>& vec, size_t size)
 template<typename Stream>
 void WriteFixedVarIntsBitSet(Stream& s, const std::vector<bool>& vec, size_t size)
 {
-    int32_t last = -1;
-    for (int32_t i = 0; i < (int32_t)vec.size(); i++) {
+    std::optional<size_t> last;
+    for (size_t i = 0; i < vec.size(); i++) {
         if (vec[i]) {
-            WriteVarInt<Stream, VarIntMode::DEFAULT, uint32_t>(s, (uint32_t)(i - last));
+            WriteVarInt<Stream, VarIntMode::DEFAULT, uint32_t>(s, static_cast<uint32_t>(last ? (i - *last) : (i + 1)));
             last = i;
         }
     }
@@ -474,17 +475,17 @@ void ReadFixedVarIntsBitSet(Stream& s, std::vector<bool>& vec, size_t size)
 {
     vec.assign(size, false);
 
-    int32_t last = -1;
+    std::optional<size_t> last;
     while(true) {
         uint32_t offset = ReadVarInt<Stream, VarIntMode::DEFAULT, uint32_t>(s);
         if (offset == 0) {
             break;
         }
-        int32_t idx = last + offset;
-        if (idx >= int32_t(size)) {
+        size_t idx = last ? (*last + offset) : (static_cast<size_t>(offset) - 1);
+        if (idx >= size) {
             throw std::ios_base::failure("out of bounds index");
         }
-        if (last != -1 && idx <= last) {
+        if (last.has_value() && idx <= *last) {
             throw std::ios_base::failure("offset overflow");
         }
         vec[idx] = true;