diff --git a/Source/cmCTest.cxx b/Source/cmCTest.cxx
index bca75406bd..4254e2ec71 100644
--- a/Source/cmCTest.cxx
+++ b/Source/cmCTest.cxx
@@ -841,6 +841,7 @@ bool cmCTest::OpenOutputFile(const std::string& path, const std::string& name,
     }
   }
   std::string filename = testingDir + "/" + name;
+  stream.SetTempExt("tmp");
   stream.Open(filename);
   if (!stream) {
     cmCTestLog(this, ERROR_MESSAGE,