diff --git a/fs/bcachefs/alloc_background.c b/fs/bcachefs/alloc_background.c
index 5c60f956b598..a012fffb1dc4 100644
--- a/fs/bcachefs/alloc_background.c
+++ b/fs/bcachefs/alloc_background.c
@@ -242,7 +242,7 @@ int bch2_alloc_v4_invalid(const struct bch_fs *c, struct bkey_s_c k,
 	struct bkey_s_c_alloc_v4 a = bkey_s_c_to_alloc_v4(k);
 
 	if (alloc_v4_u64s(a.v) > bkey_val_u64s(k.k)) {
-		prt_printf(err, "bad val size (%u > %lu)",
+		prt_printf(err, "bad val size (%u > %zu)",
 		       alloc_v4_u64s(a.v), bkey_val_u64s(k.k));
 		return -BCH_ERR_invalid_bkey;
 	}