diff --git a/Modules/CTest.cmake b/Modules/CTest.cmake
index 76f256877..7b737fa8d 100644
--- a/Modules/CTest.cmake
+++ b/Modules/CTest.cmake
@@ -18,19 +18,26 @@ IF(BUILD_TESTING)
     ENDIF("${val}" MATCHES "^$")
   ENDMACRO(SET_IF_SET)
 
+  MACRO(SET_IF_SET_AND_NOT_SET var val)
+    IF("${val}" MATCHES "^$")
+    ELSE("${val}" MATCHES "^$")
+      SET_IF_NOT_SET("${var}" "${val}")
+    ENDIF("${val}" MATCHES "^$")
+  ENDMACRO(SET_IF_SET_AND_NOT_SET)
+
   # Make sure testing is enabled
   ENABLE_TESTING()
 
   IF(EXISTS "${PROJECT_SOURCE_DIR}/CTestConfig.cmake")
     INCLUDE("${PROJECT_SOURCE_DIR}/CTestConfig.cmake")
-    SET_IF_SET(NIGHTLY_START_TIME "${CTEST_NIGHTLY_START_TIME}")
-    SET_IF_SET(DROP_METHOD "${CTEST_DROP_METHOD}")
-    SET_IF_SET(DROP_SITE "${CTEST_DROP_SITE}")
-    SET_IF_SET(DROP_SITE_USER "${CTEST_DROP_SITE_USER}")
-    SET_IF_SET(DROP_SITE_PASSWORD "${CTEST_DROP_SITE_PASWORD}")
-    SET_IF_SET(DROP_SITE_MODE "${CTEST_DROP_SITE_MODE}")
-    SET_IF_SET(DROP_LOCATION "${CTEST_DROP_LOCATION}")
-    SET_IF_SET(TRIGGER_SITE "${CTEST_TRIGGER_SITE}")
+    SET_IF_SET_AND_NOT_SET(NIGHTLY_START_TIME "${CTEST_NIGHTLY_START_TIME}")
+    SET_IF_SET_AND_NOT_SET(DROP_METHOD "${CTEST_DROP_METHOD}")
+    SET_IF_SET_AND_NOT_SET(DROP_SITE "${CTEST_DROP_SITE}")
+    SET_IF_SET_AND_NOT_SET(DROP_SITE_USER "${CTEST_DROP_SITE_USER}")
+    SET_IF_SET_AND_NOT_SET(DROP_SITE_PASSWORD "${CTEST_DROP_SITE_PASWORD}")
+    SET_IF_SET_AND_NOT_SET(DROP_SITE_MODE "${CTEST_DROP_SITE_MODE}")
+    SET_IF_SET_AND_NOT_SET(DROP_LOCATION "${CTEST_DROP_LOCATION}")
+    SET_IF_SET_AND_NOT_SET(TRIGGER_SITE "${CTEST_TRIGGER_SITE}")
   ENDIF(EXISTS "${PROJECT_SOURCE_DIR}/CTestConfig.cmake")
 
   # the project can have a DartConfig.cmake file