diff --git a/Source/cmCTest.cxx b/Source/cmCTest.cxx
index e9e84b513..1e079b923 100644
--- a/Source/cmCTest.cxx
+++ b/Source/cmCTest.cxx
@@ -1070,7 +1070,7 @@ int cmCTest::RunTest(std::vector<const char*> argv,
     }
 
   cmsysProcess_WaitForExit(cp, 0);
-  if(output)
+  if(output && tempOutput.begin() != tempOutput.end())
     {
     output->append(&*tempOutput.begin(), tempOutput.size());
     }
diff --git a/Source/cmSystemTools.cxx b/Source/cmSystemTools.cxx
index 0621ba22b..623a0f917 100644
--- a/Source/cmSystemTools.cxx
+++ b/Source/cmSystemTools.cxx
@@ -507,7 +507,7 @@ bool cmSystemTools::RunSingleCommand(
     }
   
   cmsysProcess_WaitForExit(cp, 0);
-  if ( output )
+  if ( output && tempOutput.begin() != tempOutput.end())
     {
     output->append(&*tempOutput.begin(), tempOutput.size());
     }