diff --git a/scan_engine.cc b/scan_engine.cc
index 638da1c5f..bca714b7d 100644
--- a/scan_engine.cc
+++ b/scan_engine.cc
@@ -153,7 +153,7 @@ extern "C" int g_has_npcap_loopback;
 #endif
 
 
-int HssPredicate::operator() (HostScanStats *lhs, HostScanStats *rhs) {
+const int HssPredicate::operator() (const HostScanStats *lhs, const HostScanStats *rhs) const {
   const struct sockaddr_storage *lss, *rss;
   lss = (lhs) ? lhs->target->TargetSockAddr() : ss;
   rss = (rhs) ? rhs->target->TargetSockAddr() : ss;
diff --git a/scan_engine.h b/scan_engine.h
index a7fbd91a3..9424611e6 100644
--- a/scan_engine.h
+++ b/scan_engine.h
@@ -653,7 +653,7 @@ struct ultra_scan_performance_vars : public scan_performance_vars {
 
 struct HssPredicate {
 public:
-  int operator() (HostScanStats *lhs, HostScanStats *rhs);
+  const int operator() (const HostScanStats *lhs, const HostScanStats *rhs) const;
   static struct sockaddr_storage *ss;
 };