diff --git a/ext/digest/extconf.rb b/ext/digest/extconf.rb
index d8b7715..aad3216 100755
--- a/ext/digest/extconf.rb
+++ b/ext/digest/extconf.rb
@@ -20,6 +20,8 @@ end
 cflags '-fvisibility=hidden'
 
 have_header! 'ruby/digest.h'
+have_header! 'stdio.h'
+have_header! 'string.h'
 
 have_func! 'rb_str_set_len'