diff --git a/libcxx/CMakeLists.txt b/libcxx/CMakeLists.txt
index 9670d93e3d98..b28dd00f6159 100644
--- a/libcxx/CMakeLists.txt
+++ b/libcxx/CMakeLists.txt
@@ -263,7 +263,7 @@ option(LIBCXXABI_USE_LLVM_UNWINDER "Build and use the LLVM unwinder." OFF)
 # Target options --------------------------------------------------------------
 option(LIBCXX_BUILD_32_BITS "Build 32 bit multilib libc++. This option is not supported anymore when building the runtimes. Please specify a full triple instead." ${LLVM_BUILD_32_BITS})
 if (LIBCXX_BUILD_32_BITS)
-  message(WARNING "LIBCXX_BUILD_32_BITS is not supported anymore when building the runtimes, please specify a full triple instead.")
+  message(FATAL_ERROR "LIBCXX_BUILD_32_BITS is not supported anymore when building the runtimes, please specify a full triple instead.")
 endif()
 
 if(CMAKE_CXX_COMPILER_TARGET)
diff --git a/libcxxabi/CMakeLists.txt b/libcxxabi/CMakeLists.txt
index 0d513884ed4d..12bcd2eee099 100644
--- a/libcxxabi/CMakeLists.txt
+++ b/libcxxabi/CMakeLists.txt
@@ -110,7 +110,7 @@ option(LIBCXXABI_ENABLE_NEW_DELETE_DEFINITIONS
    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})
diff --git a/libunwind/CMakeLists.txt b/libunwind/CMakeLists.txt
index 3727d1159836..a63dc453ffb6 100644
--- a/libunwind/CMakeLists.txt
+++ b/libunwind/CMakeLists.txt
@@ -59,7 +59,7 @@ include(HandleCompilerRT)
 # Define options.
 option(LIBUNWIND_BUILD_32_BITS "Build 32 bit multilib libunwind. This option is not supported anymore when building the runtimes. Please specify a full triple instead." ${LLVM_BUILD_32_BITS})
 if (LIBUNWIND_BUILD_32_BITS)
-  message(WARNING "LIBUNWIND_BUILD_32_BITS is not supported anymore when building the runtimes, please specify a full triple instead.")
+  message(FATAL_ERROR "LIBUNWIND_BUILD_32_BITS is not supported anymore when building the runtimes, please specify a full triple instead.")
 endif()
 
 option(LIBUNWIND_ENABLE_CET "Build libunwind with CET enabled." OFF)