diff --git a/include/ruby/io.h b/include/ruby/io.h
index 8f0367707e..b1e94361c3 100644
--- a/include/ruby/io.h
+++ b/include/ruby/io.h
@@ -51,12 +51,12 @@ extern "C" {
 
 RUBY_SYMBOL_EXPORT_BEGIN
 
-typedef PACKED_STRUCT(struct {
+PACKED_STRUCT(typedef struct {
     char *ptr;                  /* off + len <= capa */
     int off;
     int len;
     int capa;
-}) rb_io_buffer_t;
+} rb_io_buffer_t);
 
 typedef struct rb_io_t {
     FILE *stdio_file;		/* stdio ptr for read/write if available */