diff --git a/Source/cmGlobalNinjaGenerator.cxx b/Source/cmGlobalNinjaGenerator.cxx
index 23f42e440..b405905f7 100644
--- a/Source/cmGlobalNinjaGenerator.cxx
+++ b/Source/cmGlobalNinjaGenerator.cxx
@@ -558,7 +558,7 @@ void cmGlobalNinjaGenerator::AddRule(const std::string& name,
                                     restat,
                                     generator);
 
-  this->RuleCmdLength[name] = command.size();
+  this->RuleCmdLength[name] = (int) command.size();
 }
 
 bool cmGlobalNinjaGenerator::HasRule(const std::string &name)