diff --git a/src/App/Document.cpp b/src/App/Document.cpp
index c808d70062..ddfad43c81 100644
--- a/src/App/Document.cpp
+++ b/src/App/Document.cpp
@@ -2303,7 +2303,8 @@ std::vector<App::DocumentObject*> DocumentP::partialTopologicalSort(const std::v
 
             for (auto outListIt : out) {
                 auto outListMapIt = countMap.find(outListIt);
-                outListMapIt->second.first = outListMapIt->second.first - 1;
+                if (outListMapIt != countMap.end())
+                    outListMapIt->second.first = outListMapIt->second.first - 1;
             }
         }
     }
@@ -2337,7 +2338,8 @@ std::vector<App::DocumentObject*> DocumentP::partialTopologicalSort(const std::v
 
             for (auto inListIt : in) {
                 auto inListMapIt = countMap.find(inListIt);
-                inListMapIt->second.second = inListMapIt->second.second - 1;
+                if (inListMapIt != countMap.end())
+                    inListMapIt->second.second = inListMapIt->second.second - 1;
             }
         }
     }