diff --git a/Tests/CTestUpdateHG.cmake.in b/Tests/CTestUpdateHG.cmake.in
index 5a9daae09a..640d2c6f3d 100644
--- a/Tests/CTestUpdateHG.cmake.in
+++ b/Tests/CTestUpdateHG.cmake.in
@@ -28,7 +28,10 @@ run_child(
   WORKING_DIRECTORY ${TOP}/repo.hg
   COMMAND ${HG} init
   )
-set(REPO file:///${TOP}/repo.hg)
+if(NOT "${TOP}" MATCHES "^/")
+  set(slash /)
+endif()
+set(REPO file://${slash}${TOP}/repo.hg)
 
 #-----------------------------------------------------------------------------
 # Import initial content into the repository.