diff --git a/flang/CMakeLists.txt b/flang/CMakeLists.txt
--- a/flang/CMakeLists.txt
+++ b/flang/CMakeLists.txt
@@ -127,6 +127,10 @@
   endif()
 
 else()
+  if(NOT "mlir" IN_LIST LLVM_ENABLE_PROJECTS)
+    message(FATAL_ERROR "'mlir' is required in LLVM_ENABLE_PROJECTS to build flang")
+  endif()
+
   option(FLANG_INCLUDE_TESTS
          "Generate build targets for the Flang unit tests."
          ${LLVM_INCLUDE_TESTS})