diff --git a/cpp/map_closures/MapClosures.cpp b/cpp/map_closures/MapClosures.cpp
index a6f8c62..85d34dd 100644
--- a/cpp/map_closures/MapClosures.cpp
+++ b/cpp/map_closures/MapClosures.cpp
@@ -110,7 +110,12 @@ void MapClosures::MatchAndAddToDatabase(const int id,
 }
 
 ClosureCandidate MapClosures::ValidateClosure(const int reference_id, const int query_id) const {
-    const Tree::MatchVector &matches = descriptor_matches_.at(reference_id);
+    const auto it = descriptor_matches_.find(reference_id);
+    if (it == descriptor_matches_.end()) {
+        return ClosureCandidate();
+    }
+
+    const Tree::MatchVector &matches = it->second;
     const size_t num_matches = matches.size();
 
     ClosureCandidate closure;