diff --git a/mytools/voidTree.hpp b/mytools/voidTree.hpp
index cafceac..3f483ae 100644
--- a/mytools/voidTree.hpp
+++ b/mytools/voidTree.hpp
@@ -87,6 +87,9 @@ public:
 
         if (candidate_zIds.size() < ref_void.zId.size())
           {
+            if (iter_candidate == candidateList.begin())
+              return -1;
+
             --iter_candidate;
             continue;
           }