diff --git a/ext/zlib/extconf.rb b/ext/zlib/extconf.rb
index 2187ed1..a405317 100644
--- a/ext/zlib/extconf.rb
+++ b/ext/zlib/extconf.rb
@@ -82,9 +82,7 @@
   Logging::message 'checking for kind of operating system... '
   os_code = with_config('os-code') ||
     case RUBY_PLATFORM.split('-',2)[1]
-    when 'amigaos' then
-      os_code = 'AMIGA'
-    when /mswin|mingw|bccwin/ then
+    when /mswin|mingw/ then
       # NOTE: cygwin should be regarded as Unix.
       os_code = 'WIN32'
     else