diff --git a/src/PTQ/PTQ.cpp b/src/PTQ/PTQ.cpp
index 76fe8f24b3492a9488f808d54c2ce6b491a9b79a..4f88aedf2724fefb041b972214e4e7172afefada 100644
--- a/src/PTQ/PTQ.cpp
+++ b/src/PTQ/PTQ.cpp
@@ -699,8 +699,12 @@ std::map<std::string, std::pair<bool, bool>> computeSignMap(std::shared_ptr<Grap
         {
             // Thoses nodes always have a single parent 
             std::shared_ptr<Node> parent = node->getParent(0); 
-            signMap[node->name()].first = signMap[parent->name()].second;
-            signMap[node->name()].second = signMap[node->name()].first;
+            if (parent)
+            {
+                signMap[node->name()].first = signMap[parent->name()].second;
+                signMap[node->name()].second = signMap[node->name()].first;
+            }
+            
         }
     }