diff --git a/llvm/CMakeLists.txt b/llvm/CMakeLists.txt
index c637febce1c1f..330db65e85cab 100644
--- a/llvm/CMakeLists.txt
+++ b/llvm/CMakeLists.txt
@@ -161,6 +161,12 @@ foreach(proj IN LISTS LLVM_ENABLE_RUNTIMES)
   endif()
 endforeach()
 
+foreach(proj IN LISTS LLVM_ENABLE_RUNTIMES)
+  if ("${proj}" IN_LIST LLVM_ENABLE_PROJECTS)
+    message(FATAL_ERROR "Runtime project \"${proj}\" found in LLVM_ENABLE_PROJECTS and LLVM_ENABLE_RUNTIMES. It must only appear in one of them and that one should almost always be LLVM_ENABLE_RUNTIMES.")
+  endif()
+endforeach()
+
 # Set a shorthand option to enable the GPU build of the 'libc' project.
 option(LIBC_GPU_BUILD "Enable the 'libc' project targeting the GPU" OFF)
 if(LIBC_GPU_BUILD)