diff --git a/ext/zlib/extconf.rb b/ext/zlib/extconf.rb
index 1254c62..dd17986 100644
--- a/ext/zlib/extconf.rb
+++ b/ext/zlib/extconf.rb
@@ -120,12 +120,16 @@
     $defs << "-DHAVE_CRC32_COMBINE"
     $defs << "-DHAVE_ADLER32_COMBINE"
     $defs << "-DHAVE_TYPE_Z_CRC_T"
-    $defs << "-DHAVE_TYPE_Z_SIZE_T"
+    $defs << "-DHAVE_CRC32_Z"
+    $defs << "-DHAVE_ADLER32_Z"
+    $defs << "-DHAVE_ZLIB_SIZE_T_FUNCS"
   else
     have_func('crc32_combine', 'zlib.h')
     have_func('adler32_combine', 'zlib.h')
     have_type('z_crc_t', 'zlib.h')
-    have_type('z_size_t', 'zlib.h')
+    if have_func('crc32_z', 'zlib.h') && have_func('adler32_z', 'zlib.h')
+      $defs << "-DHAVE_ZLIB_SIZE_T_FUNCS"
+    end
   end
 
   create_makefile('zlib') {|conf|
diff --git a/ext/zlib/zlib.c b/ext/zlib/zlib.c
index ef6e892..292e640 100644
--- a/ext/zlib/zlib.c
+++ b/ext/zlib/zlib.c
@@ -44,7 +44,7 @@
 #endif
 #endif
 
-#if defined(HAVE_TYPE_Z_SIZE_T)
+#if defined(HAVE_ZLIB_SIZE_T_FUNCS)
 typedef uLong (*checksum_func)(uLong, const Bytef*, z_size_t);
 # define crc32 crc32_z
 # define adler32 adler32_z
@@ -388,7 +388,7 @@ rb_zlib_version(VALUE klass)
 # define mask32(x) (x)
 #endif
 
-#if SIZEOF_LONG > SIZEOF_INT && !defined(HAVE_TYPE_Z_SIZE_T)
+#if SIZEOF_LONG > SIZEOF_INT && !defined(HAVE_ZLIB_SIZE_T_FUNCS)
 static uLong
 checksum_long(uLong (*func)(uLong, const Bytef*, uInt), uLong sum, const Bytef *ptr, long len)
 {