diff --git a/traceroute.cc b/traceroute.cc
index 7d19f4169..a7d244efd 100644
--- a/traceroute.cc
+++ b/traceroute.cc
@@ -1176,7 +1176,7 @@ TraceGroup::retransmissions(vector < TraceProbe * >&retrans) {
 
 /* Returns a map from TTLs to probes, stripped of all unneeded probes and with
  * timed-out probes marked for consolidation. */
-map < u8, TraceProbe * > TraceGroup::consolidateHops() {
+map < u8, TraceProbe * > TraceGroup::consolidateHops() const {
     map < u16, TraceProbe * >::size_type ttl_count;
     map < u8, TraceProbe * >ttlProbes;
     map < u16, TraceProbe * >::const_iterator probe_iter;
diff --git a/traceroute.h b/traceroute.h
index 00f599b16..668ec24b6 100644
--- a/traceroute.h
+++ b/traceroute.h
@@ -210,7 +210,7 @@ class TraceGroup {
      void retransmissions(std::vector < TraceProbe * >&retrans);
     /* Returns a map from TTLs to probes, stripped of all unneeded probes and
      * with timed-out probes marked for consolidation. */
-    std::map < u8, TraceProbe * > consolidateHops();
+    std::map < u8, TraceProbe * > consolidateHops() const;
     /* the next ttl to send, if the destination has replied the ttl is
      * decremented, if it hasn't it is incremented */
     void nextTTL();