diff --git a/Source/cmGeneratorExpressionDAGChecker.cxx b/Source/cmGeneratorExpressionDAGChecker.cxx
index 728f2a4fd6..817f41e653 100644
--- a/Source/cmGeneratorExpressionDAGChecker.cxx
+++ b/Source/cmGeneratorExpressionDAGChecker.cxx
@@ -68,9 +68,7 @@ void cmGeneratorExpressionDAGChecker::Initialize()
         return;
       }
     }
-    const_cast<cmGeneratorExpressionDAGChecker*>(top)
-      ->Seen[this->Target]
-      .insert(this->Property);
+    top->Seen[this->Target].insert(this->Property);
   }
 }
 
diff --git a/Source/cmGeneratorExpressionDAGChecker.h b/Source/cmGeneratorExpressionDAGChecker.h
index e1fba5e5cf..6d7d6ef140 100644
--- a/Source/cmGeneratorExpressionDAGChecker.h
+++ b/Source/cmGeneratorExpressionDAGChecker.h
@@ -88,7 +88,7 @@ private:
   const cmGeneratorExpressionDAGChecker* const Parent;
   cmGeneratorTarget const* Target;
   const std::string Property;
-  std::map<cmGeneratorTarget const*, std::set<std::string>> Seen;
+  mutable std::map<cmGeneratorTarget const*, std::set<std::string>> Seen;
   const GeneratorExpressionContent* const Content;
   const cmListFileBacktrace Backtrace;
   Result CheckResult;