diff --git a/compiler-rt/lib/fuzzer/CMakeLists.txt b/compiler-rt/lib/fuzzer/CMakeLists.txt
--- a/compiler-rt/lib/fuzzer/CMakeLists.txt
+++ b/compiler-rt/lib/fuzzer/CMakeLists.txt
@@ -144,7 +144,7 @@
       CFLAGS ${TARGET_CFLAGS}
       CMAKE_ARGS -DCMAKE_CXX_COMPILER_WORKS=ON
                  -DCMAKE_POSITION_INDEPENDENT_CODE=ON
-                 -DLIBCXX_ABI_NAMESPACE=Fuzzer)
+                 -DLIBCXX_ABI_NAMESPACE=__Fuzzer)
     target_compile_options(RTfuzzer.${arch} PRIVATE -isystem ${LIBCXX_${arch}_PREFIX}/include/c++/v1)
     add_dependencies(RTfuzzer.${arch} libcxx_fuzzer_${arch}-build)
     target_compile_options(RTfuzzer_main.${arch} PRIVATE -isystem ${LIBCXX_${arch}_PREFIX}/include/c++/v1)
diff --git a/libcxx/CMakeLists.txt b/libcxx/CMakeLists.txt
--- a/libcxx/CMakeLists.txt
+++ b/libcxx/CMakeLists.txt
@@ -816,7 +816,7 @@
 endif()
 if (NOT LIBCXX_ABI_NAMESPACE STREQUAL "")
   if (NOT LIBCXX_ABI_NAMESPACE MATCHES "__.*")
-    message(WARNING "LIBCXX_ABI_NAMESPACE must be a reserved identifier.")
+    message(FATAL_ERROR "LIBCXX_ABI_NAMESPACE must be a reserved identifier.")
   endif()
   if (LIBCXX_ABI_NAMESPACE MATCHES "__[0-9]+$")
     message(FATAL_ERROR "LIBCXX_ABI_NAMESPACE '${LIBCXX_ABI_NAMESPACE}' is reserved for use by libc++.")