diff --git a/Templates/TestDriver.cxx.in b/Templates/TestDriver.cxx.in
index f4510bb55..03916bf74 100644
--- a/Templates/TestDriver.cxx.in
+++ b/Templates/TestDriver.cxx.in
@@ -137,6 +137,13 @@ int main(int ac, char *av[])
     {
     int result;
 @CMAKE_TESTDRIVER_BEFORE_TESTMAIN@
+    if (testToRun < 0 || testToRun >= NumTests)
+      {
+      printf(
+        "testToRun was modified by TestDriver code to an invalid value: %3d.\n",
+        testNum);
+      return -1;
+      }
     result = (*cmakeGeneratedFunctionMapEntries[testToRun].func)(ac, av);
 @CMAKE_TESTDRIVER_AFTER_TESTMAIN@
     return result;