diff --git a/CMakeLists.txt b/CMakeLists.txt
index 9a11ad55..3a9af151 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,5 +1,7 @@
 cmake_minimum_required(VERSION 3.13)
-
+# CMP0025: Compiler id for Apple Clang is now AppleClang.
+# https://cmake.org/cmake/help/v3.17/policy/CMP0025.html
+cmake_policy(SET CMP0025 NEW)
 project(simdjson
   DESCRIPTION "Parsing gigabytes of JSON per second"
   LANGUAGES CXX C
diff --git a/cmake/simdjson-flags.cmake b/cmake/simdjson-flags.cmake
index ecffc65d..13a266a7 100644
--- a/cmake/simdjson-flags.cmake
+++ b/cmake/simdjson-flags.cmake
@@ -16,18 +16,34 @@ add_library(simdjson-flags INTERFACE)
 add_library(simdjson-internal-flags INTERFACE)
 target_link_libraries(simdjson-internal-flags INTERFACE simdjson-flags)
 
+option(SIMDJSON_SANITIZE_UNDEFINED "Sanitize undefined behavior" OFF)
+if(SIMDJSON_SANITIZE_UNDEFINED)
+  target_compile_options(simdjson-flags INTERFACE -fsanitize=undefined -fno-sanitize-recover=all)
+  target_link_libraries(simdjson-flags INTERFACE -fsanitize=undefined -fno-sanitize-recover=all)
+endif()
+
 option(SIMDJSON_SANITIZE "Sanitize addresses" OFF)
 if(SIMDJSON_SANITIZE)
-  target_compile_options(simdjson-flags INTERFACE -fsanitize=address -fno-omit-frame-pointer -fsanitize=undefined -fno-sanitize-recover=all)
-  target_link_libraries(simdjson-flags INTERFACE -fsanitize=address -fno-omit-frame-pointer -fsanitize=undefined -fno-sanitize-recover=all)
-
+  if(CMAKE_CXX_COMPILER_ID STREQUAL "AppleClang")
+    message(STATUS "The address sanitizer under Apple's clang appears to be incompatible with the undefined-behavior sanitizer.")
+    message(STATUS "You may set SIMDJSON_SANITIZE_UNDEFINED to sanitize undefined behavior.")
+    target_compile_options(simdjson-flags INTERFACE -fsanitize=address  -fno-omit-frame-pointer -fno-sanitize-recover=all)
+    target_compile_definitions(simdjson-flags INTERFACE ASAN_OPTIONS=detect_leaks=1)
+    target_link_libraries(simdjson-flags INTERFACE -fsanitize=address  -fno-omit-frame-pointer -fno-sanitize-recover=all)
+  else()
+    message(STATUS "Setting both the address sanitizer and the undefined sanitizer.")
+    target_compile_options(simdjson-flags INTERFACE -fsanitize=address -fno-omit-frame-pointer -fsanitize=undefined -fno-sanitize-recover=all)
+    target_link_libraries(simdjson-flags INTERFACE -fsanitize=address -fno-omit-frame-pointer -fsanitize=undefined -fno-sanitize-recover=all)
+  endif()
   # Ubuntu bug for GCC 5.0+ (safe for all versions)
   if (CMAKE_COMPILER_IS_GNUCC)
     target_link_libraries(simdjson-flags INTERFACE -fuse-ld=gold)
   endif()
 endif()
 
+
 if(SIMDJSON_SANITIZE_THREADS)
+  message(STATUS "Setting both the thread sanitizer and the undefined-behavior sanitizer.")
   target_compile_options(simdjson-flags INTERFACE -fsanitize=thread -fsanitize=undefined -fno-sanitize-recover=all)
   target_link_libraries(simdjson-flags INTERFACE -fsanitize=thread -fsanitize=undefined -fno-sanitize-recover=all)
 
@@ -38,10 +54,13 @@ if(SIMDJSON_SANITIZE_THREADS)
 endif()
 
 if (NOT CMAKE_BUILD_TYPE)
-  message(STATUS "No build type selected, default to Release")
-  set(CMAKE_BUILD_TYPE Release CACHE STRING "Choose the type of build." FORCE)
-  if(SIMDJSON_SANITIZE)
-    message(WARNING "No build type selected and you have enabled the sanitizer. Consider setting CMAKE_BUILD_TYPE to Debug to help identify the eventual problems.")
+  # Deliberately not including SIMDJSON_SANITIZE_THREADS since thread behavior depends on the build type.
+  if(SIMDJSON_SANITIZE OR SIMDJSON_SANITIZE_UNDEFINED)
+    message(STATUS "No build type selected and you have enabled the sanitizer, default to Debug. Consider setting CMAKE_BUILD_TYPE.")
+    set(CMAKE_BUILD_TYPE Debug CACHE STRING "Choose the type of build." FORCE)
+  else()
+    message(STATUS "No build type selected, default to Release")
+    set(CMAKE_BUILD_TYPE Release CACHE STRING "Choose the type of build." FORCE)
   endif()
 endif()