diff --git a/src/mempoollimit.cpp b/src/mempoollimit.cpp
index 4c42d6044..594154d31 100644
--- a/src/mempoollimit.cpp
+++ b/src/mempoollimit.cpp
@@ -59,7 +59,7 @@ void WeightedTxTree::backPropagate(size_t fromIndex, const TxWeight& weightDelta
     }
 }
 
-size_t WeightedTxTree::findByWeight(size_t fromIndex, uint64_t weightToFind) const
+size_t WeightedTxTree::findByWeight(size_t fromIndex, int64_t weightToFind) const
 {
     int leftWeight = getWeightAt(fromIndex * 2 + 1).lowFeePenaltyWeight;
     int rightWeight = getWeightAt(fromIndex).lowFeePenaltyWeight - getWeightAt(fromIndex * 2 + 2).lowFeePenaltyWeight;
@@ -124,7 +124,7 @@ void WeightedTxTree::remove(const uint256& txId)
 
 boost::optional<uint256> WeightedTxTree::maybeDropRandom()
 {
-    uint64_t totalPenaltyWeight = getTotalWeight().lowFeePenaltyWeight;
+    int64_t totalPenaltyWeight = getTotalWeight().lowFeePenaltyWeight;
     if (totalPenaltyWeight <= capacity) {
         return boost::none;
     }
@@ -155,8 +155,8 @@ WeightedTxInfo WeightedTxInfo::from(const CTransaction& tx, const CAmount& fee)
     memUsage += tx.vJoinSplit.size() * JOINSPLIT_SIZE;
     memUsage += tx.vShieldedOutput.size() * OUTPUTDESCRIPTION_SIZE;
     memUsage += tx.vShieldedSpend.size() * SPENDDESCRIPTION_SIZE;
-    uint64_t cost = std::max(memUsage, MIN_TX_WEIGHT);
-    uint64_t lowFeePenaltyCost = cost;
+    int64_t cost = std::max(memUsage, MIN_TX_WEIGHT);
+    int64_t lowFeePenaltyCost = cost;
     if (fee < DEFAULT_FEE) {
         lowFeePenaltyCost += LOW_FEE_PENALTY;
     }
diff --git a/src/mempoollimit.h b/src/mempoollimit.h
index af7b93f1f..985d1db55 100644
--- a/src/mempoollimit.h
+++ b/src/mempoollimit.h
@@ -46,10 +46,10 @@ public:
 
 
 struct TxWeight {
-    uint64_t weight;
-    uint64_t lowFeePenaltyWeight;
+    int64_t weight;
+    int64_t lowFeePenaltyWeight;
 
-    TxWeight(uint64_t weight_, uint64_t lowFeePenaltyWeight_)
+    TxWeight(int64_t weight_, int64_t lowFeePenaltyWeight_)
         : weight(weight_), lowFeePenaltyWeight(lowFeePenaltyWeight_) {}
 
     TxWeight add(const TxWeight& other) const;
@@ -69,7 +69,7 @@ struct WeightedTxInfo {
 
 class WeightedTxTree
 {
-    const uint64_t capacity;
+    const int64_t capacity;
     size_t size = 0;
     
     std::vector<WeightedTxInfo> txIdAndWeights;
@@ -78,10 +78,12 @@ class WeightedTxTree
 
     TxWeight getWeightAt(size_t index) const;
     void backPropagate(size_t fromIndex, const TxWeight& weightDelta);
-    size_t findByWeight(size_t fromIndex, uint64_t weightToFind) const;
+    size_t findByWeight(size_t fromIndex, int64_t weightToFind) const;
 
 public:
-    WeightedTxTree(uint64_t capacity_) : capacity(capacity_) {}
+    WeightedTxTree(int64_t capacity_) : capacity(capacity_) {
+        assert(capacity >= 0);
+    }
 
     TxWeight getTotalWeight() const;