diff --git a/llvm/CMakeLists.txt b/llvm/CMakeLists.txt
--- a/llvm/CMakeLists.txt
+++ b/llvm/CMakeLists.txt
@@ -68,9 +68,15 @@
 # The flang project is not yet part of "all" projects (see C++ requirements)
 set(LLVM_EXTRA_PROJECTS "flang")
 # List of all known projects in the mono repo
-set(LLVM_KNOWN_PROJECTS "${LLVM_ALL_PROJECTS};${LLVM_EXTRA_PROJECTS}")
+set(LLVM_KNOWN_PROJECTS "${LLVM_ALL_PROJECTS};${LLVM_EXTRA_PROJECTS};llvm")
 set(LLVM_ENABLE_PROJECTS "" CACHE STRING
 	"Semicolon-separated list of projects to build (${LLVM_KNOWN_PROJECTS}), or \"all\".")
+foreach(proj ${LLVM_ENABLE_PROJECTS})
+  if (NOT "${proj}" IN_LIST LLVM_KNOWN_PROJECTS)
+     MESSAGE(FATAL_ERROR "${proj} isn't a know project: ${LLVM_KNOWN_PROJECTS}")
+  endif()
+endforeach()
+
 if( LLVM_ENABLE_PROJECTS STREQUAL "all" )
   set( LLVM_ENABLE_PROJECTS ${LLVM_ALL_PROJECTS})
 endif()