diff --git a/src/main.cpp b/src/main.cpp
index cd3d63c25e0f..7b3baa77e7da 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -94,7 +94,7 @@ FeeFilterRounder filterRounder(::minRelayTxFee);
 struct IteratorComparator
 {
     template<typename I>
-    bool operator()(const I& a, const I& b)
+    bool operator()(const I& a, const I& b) const
     {
         return &(*a) < &(*b);
     }