diff --git a/Tests/RunCMake/Ninja/CheckNoPrefixSubDirScript.cmake b/Tests/RunCMake/Ninja/CheckNoPrefixSubDirScript.cmake
index 5a03fcb7e6..09fd7e9b5f 100644
--- a/Tests/RunCMake/Ninja/CheckNoPrefixSubDirScript.cmake
+++ b/Tests/RunCMake/Ninja/CheckNoPrefixSubDirScript.cmake
@@ -1,6 +1,6 @@
 # Check that the prefix sub-directory is not repeated.
 
-if(EXISTS "${CUR_BIN_DIR}/${NINJA_OUTPUT_PATH_PREFIX}")
+if(NINJA_OUTPUT_PATH_PREFIX AND EXISTS "${CUR_BIN_DIR}/${NINJA_OUTPUT_PATH_PREFIX}")
   message(FATAL_ERROR
     "no sub directory named after the CMAKE_NINJA_OUTPUT_PATH_PREFIX "
     "should be in the binary directory."