diff --git a/io/iob_addbuf_munmap.c b/io/iob_addbuf_munmap.c
index 10b09b0..86bd370 100644
--- a/io/iob_addbuf_munmap.c
+++ b/io/iob_addbuf_munmap.c
@@ -3,7 +3,7 @@
 #include "iob_internal.h"
 
 static void cleanup(struct iob_entry* x) {
-  munmap((char*)x->buf,x->n);
+  munmap((char*)x->buf,x->offset+x->n);
 }
 
 int iob_addbuf_munmap(io_batch* b,const void* buf,uint64 n) {