diff --git a/bin/mpirunDebug b/bin/mpirunDebug
index 151eedd5c4..b7fb377f0f 100755
--- a/bin/mpirunDebug
+++ b/bin/mpirunDebug
@@ -239,6 +239,11 @@ MPICH)
         cmd="${cmd} -n 1 ${procXtermCmdFile}"
     done < $PWD/mpirun.schema
     ;;
+*)
+    echo
+    echo "Unsupported WM_MPLIB setting : $WM_MPLIB"
+    printUsage
+    exit 1
 esac
 
 echo "Constructed $PWD/mpirun.schema file."