diff --git a/CMakeLists.txt b/CMakeLists.txt
index 0d51388..12bcd2e 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -110,7 +110,7 @@
    which case the definition in libc++abi should be turned off." ON)
 option(LIBCXXABI_BUILD_32_BITS "Build 32 bit multilib libc++abi. This option is not supported anymore when building the runtimes. Please specify a full triple instead." ${LLVM_BUILD_32_BITS})
 if (LIBCXXABI_BUILD_32_BITS)
-  message(WARNING "LIBCXXABI_BUILD_32_BITS is not supported anymore when building the runtimes, please specify a full triple instead.")
+  message(FATAL_ERROR "LIBCXXABI_BUILD_32_BITS is not supported anymore when building the runtimes, please specify a full triple instead.")
 endif()
 
 option(LIBCXXABI_INCLUDE_TESTS "Generate build targets for the libc++abi unit tests." ${LLVM_INCLUDE_TESTS})