diff --git a/mdb.c b/mdb.c
index 8ad6ef35..e2214db9 100644
--- a/mdb.c
+++ b/mdb.c
@@ -4097,6 +4097,7 @@ mdb_env_sync0(MDB_env *env, unsigned flags, MDB_meta *pending)
 	size_t prev_mapsize = head->mm_mapsize;
 	MDB_meta* tail = META_IS_WEAK(head) ? head : mdb_env_meta_flipflop(env, head);
 	off_t offset = (char*) tail - env->me_map;
+	size_t used_size = env->me_psize * (pending->mm_last_pg + 1);
 
 	mdb_assert(env, (env->me_flags & (MDB_RDONLY | MDB_FATAL_ERROR)) == 0);
 	mdb_assert(env, META_IS_WEAK(head) || env->me_sync_pending != 0
@@ -4108,6 +4109,7 @@ mdb_env_sync0(MDB_env *env, unsigned flags, MDB_meta *pending)
 	mdb_assert(env, pending->mm_txnid > stay->mm_txnid);
 
 	pending->mm_mapsize = env->me_mapsize;
+	mdb_assert(env, pending->mm_mapsize >= used_size);
 	if (unlikely(pending->mm_mapsize != prev_mapsize)) {
 		if (pending->mm_mapsize < prev_mapsize) {
 			/* LY: currently this can't happen, but force full-sync. */
@@ -4124,7 +4126,7 @@ mdb_env_sync0(MDB_env *env, unsigned flags, MDB_meta *pending)
 	if (env->me_sync_pending && (flags & MDB_NOSYNC) == 0) {
 		if (env->me_flags & MDB_WRITEMAP) {
 			int mode = (flags & MDB_MAPASYNC) ? MS_ASYNC : MS_SYNC;
-			if (unlikely(msync(env->me_map, pending->mm_mapsize, mode))) {
+			if (unlikely(msync(env->me_map, used_size, mode))) {
 				rc = errno;
 				goto fail;
 			}