diff --git a/Tests/RunCMake/RunCMake.cmake b/Tests/RunCMake/RunCMake.cmake
index b24ae0b0e..70c0d6c37 100644
--- a/Tests/RunCMake/RunCMake.cmake
+++ b/Tests/RunCMake/RunCMake.cmake
@@ -135,3 +135,6 @@ function(run_cmake_command test)
   set(RunCMake_TEST_COMMAND "${ARGN}")
   run_cmake(${test})
 endfunction()
+
+# Protect RunCMake tests from calling environment.
+unset(ENV{MAKEFLAGS})