diff --git a/Source/cmGlobalGenerator.cxx b/Source/cmGlobalGenerator.cxx
index f90b06f59b..a50721c33a 100644
--- a/Source/cmGlobalGenerator.cxx
+++ b/Source/cmGlobalGenerator.cxx
@@ -1076,7 +1076,7 @@ void cmGlobalGenerator::AddCMP0042WarnTarget(const std::string& target)
   this->CMP0042WarnTargets.insert(target);
 }
 
-bool cmGlobalGenerator::CheckALLOW_DUPLICATE_CUSTOM_TARGETS()
+bool cmGlobalGenerator::CheckALLOW_DUPLICATE_CUSTOM_TARGETS() const
 {
   // If the property is not enabled then okay.
   if(!this->CMakeInstance
diff --git a/Source/cmGlobalGenerator.h b/Source/cmGlobalGenerator.h
index 0c84cce104..e2d7515312 100644
--- a/Source/cmGlobalGenerator.h
+++ b/Source/cmGlobalGenerator.h
@@ -334,7 +334,7 @@ protected:
 
   virtual bool ComputeTargetDepends();
 
-  virtual bool CheckALLOW_DUPLICATE_CUSTOM_TARGETS();
+  virtual bool CheckALLOW_DUPLICATE_CUSTOM_TARGETS() const;
 
   bool CheckTargets();
   typedef std::vector<std::pair<cmQtAutoGenerators,
diff --git a/Source/cmGlobalNinjaGenerator.h b/Source/cmGlobalNinjaGenerator.h
index de389237f6..0d5fb44820 100644
--- a/Source/cmGlobalNinjaGenerator.h
+++ b/Source/cmGlobalNinjaGenerator.h
@@ -304,7 +304,7 @@ protected:
 
   /// Overloaded methods.
   /// @see cmGlobalGenerator::CheckALLOW_DUPLICATE_CUSTOM_TARGETS()
-  virtual bool CheckALLOW_DUPLICATE_CUSTOM_TARGETS() { return true; }
+  virtual bool CheckALLOW_DUPLICATE_CUSTOM_TARGETS() const { return true; }
 
 
 private:
diff --git a/Source/cmGlobalUnixMakefileGenerator3.h b/Source/cmGlobalUnixMakefileGenerator3.h
index 00a86ebfea..9173751d16 100644
--- a/Source/cmGlobalUnixMakefileGenerator3.h
+++ b/Source/cmGlobalUnixMakefileGenerator3.h
@@ -161,7 +161,7 @@ protected:
   const char* GetRebuildCacheTargetName()  const { return "rebuild_cache"; }
   const char* GetCleanTargetName()         const { return "clean"; }
 
-  virtual bool CheckALLOW_DUPLICATE_CUSTOM_TARGETS() { return true; }
+  virtual bool CheckALLOW_DUPLICATE_CUSTOM_TARGETS() const { return true; }
 
   // Some make programs (Borland) do not keep a rule if there are no
   // dependencies or commands.  This is a problem for creating rules