diff --git a/llvm/CMakeLists.txt b/llvm/CMakeLists.txt
--- a/llvm/CMakeLists.txt
+++ b/llvm/CMakeLists.txt
@@ -71,6 +71,12 @@
 set(LLVM_KNOWN_PROJECTS "${LLVM_ALL_PROJECTS};${LLVM_EXTRA_PROJECTS}")
 set(LLVM_ENABLE_PROJECTS "" CACHE STRING
 	"Semicolon-separated list of projects to build (${LLVM_KNOWN_PROJECTS}), or \"all\".")
+foreach(proj ${LLVM_ENABLE_PROJECTS})
+  if (NOT proj STREQUAL "all" AND NOT proj STREQUAL "llvm" AND NOT "${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()