diff --git a/Source/cmExecuteProcessCommand.cxx b/Source/cmExecuteProcessCommand.cxx
index 1d9db3011..409052919 100644
--- a/Source/cmExecuteProcessCommand.cxx
+++ b/Source/cmExecuteProcessCommand.cxx
@@ -295,11 +295,11 @@ bool cmExecuteProcessCommand::InitialPass(std::vector<std::string> const& args)
   cmExecuteProcessCommandFixText(tempError);
 
   // Store the output obtained.
-  if(!output_variable.empty())
+  if(!output_variable.empty() && tempOutput.size())
     {
     this->Makefile->AddDefinition(output_variable.c_str(), &*tempOutput.begin());
     }
-  if(!merge_output && !error_variable.empty())
+  if(!merge_output && !error_variable.empty() && tempError.size())
     {
     this->Makefile->AddDefinition(error_variable.c_str(), &*tempError.begin());
     }