1 files changed, 1 insertions, 1 deletions
diff --git a/io.c b/io.c
index e92c88117a..8fbb6cbc99 100644
--- a/io.c
+++ b/io.c
@@ -3536,7 +3536,7 @@ rb_io_fptr_finalize(rb_io_t *fptr)
 size_t rb_econv_memsize(rb_econv_t *);
 
 size_t
-rb_io_memsize(rb_io_t *fptr)
+rb_io_memsize(const rb_io_t *fptr)
 {
     size_t size = sizeof(rb_io_t);
     size += fptr->rbuf_capa;