diff --git a/io.c b/io.c
index 3153ff7c23..16cb9b5607 100644
--- a/io.c
+++ b/io.c
@@ -8408,7 +8408,7 @@ rb_io_set_encoding(int argc, VALUE *argv, VALUE io)
 }
 
 void
-rb_stdio_set_default_encoding()
+rb_stdio_set_default_encoding(void)
 {
     extern VALUE rb_stdin, rb_stdout, rb_stderr;
     VALUE val = Qnil;
diff --git a/ruby.c b/ruby.c
index 35c7b457f5..297f0e2812 100644
--- a/ruby.c
+++ b/ruby.c
@@ -1211,7 +1211,7 @@ rb_f_chomp(argc, argv)
     return str;
 }
 
-void rb_stdio_set_default_encoding();
+void rb_stdio_set_default_encoding(void);
 
 static VALUE
 process_options(int argc, char **argv, struct cmdline_options *opt)