diff --git a/include/osmocore/msgb.h b/include/osmocore/msgb.h
index e2755ef..6dc0778 100644
--- a/include/osmocore/msgb.h
+++ b/include/osmocore/msgb.h
@@ -103,7 +103,7 @@
 }
 static inline int msgb_tailroom(const struct msgb *msgb)
 {
-	return (msgb->data + msgb->data_len) - msgb->tail;
+	return (msgb->head + msgb->data_len) - msgb->tail;
 }
 
 /* increase the headroom of an empty msgb, reducing the tailroom */