diff --git a/db.go b/db.go
index d9749f4..f911146 100644
--- a/db.go
+++ b/db.go
@@ -491,8 +491,19 @@ func (db *DB) mmap(minsz int) error {
 	return nil
 }
 
+func (db *DB) invalidate() {
+	db.dataref = nil
+	db.data = nil
+	db.datasz = 0
+
+	db.meta0 = nil
+	db.meta1 = nil
+}
+
 // munmap unmaps the data file from memory.
 func (db *DB) munmap() error {
+	defer db.invalidate()
+
 	if err := munmap(db); err != nil {
 		return fmt.Errorf("unmap error: " + err.Error())
 	}
@@ -701,6 +712,13 @@ func (db *DB) beginTx() (*Tx, error) {
 		return nil, ErrDatabaseNotOpen
 	}
 
+	// Exit if the database is not correctly mapped.
+	if db.data == nil {
+		db.mmaplock.RUnlock()
+		db.metalock.Unlock()
+		return nil, ErrInvalidMapping
+	}
+
 	// Create a transaction associated with the database.
 	t := &Tx{}
 	t.init(db)
@@ -742,6 +760,12 @@ func (db *DB) beginRWTx() (*Tx, error) {
 		return nil, ErrDatabaseNotOpen
 	}
 
+	// Exit if the database is not correctly mapped.
+	if db.data == nil {
+		db.rwlock.Unlock()
+		return nil, ErrInvalidMapping
+	}
+
 	// Create a transaction associated with the database.
 	t := &Tx{writable: true}
 	t.init(db)
@@ -1016,6 +1040,7 @@ func (db *DB) Stats() Stats {
 // This is for internal access to the raw data bytes from the C cursor, use
 // carefully, or not at all.
 func (db *DB) Info() *Info {
+	_assert(db.data != nil, "database file isn't correctly mapped")
 	return &Info{uintptr(unsafe.Pointer(&db.data[0])), db.pageSize}
 }
 
@@ -1042,7 +1067,7 @@ func (db *DB) meta() *meta {
 		metaB = db.meta0
 	}
 
-	// Use higher meta page if valid. Otherwise fallback to previous, if valid.
+	// Use higher meta page if valid. Otherwise, fallback to previous, if valid.
 	if err := metaA.validate(); err == nil {
 		return metaA
 	} else if err := metaB.validate(); err == nil {
diff --git a/db_test.go b/db_test.go
index 66b0aa3..9f1076f 100644
--- a/db_test.go
+++ b/db_test.go
@@ -1331,9 +1331,8 @@ func TestDBUnmap(t *testing.T) {
 	assert.True(t, data.IsNil())
 	assert.True(t, datasz.IsZero())
 
-	// We need to reopen the db, otherwise MustCheck may panic.
+	// Set db.DB to nil to prevent MustCheck from panicking.
 	db.DB = nil
-	db.MustReopen()
 }
 
 func ExampleDB_Update() {
diff --git a/errors.go b/errors.go
index 2ad14f1..f2c3b20 100644
--- a/errors.go
+++ b/errors.go
@@ -16,6 +16,9 @@ var (
 	// This typically occurs when a file is not a bolt database.
 	ErrInvalid = errors.New("invalid database")
 
+	// ErrInvalidMapping is returned when the database file fails to get mapped.
+	ErrInvalidMapping = errors.New("database isn't correctly mapped")
+
 	// ErrVersionMismatch is returned when the data file was created with a
 	// different version of Bolt.
 	ErrVersionMismatch = errors.New("version mismatch")
diff --git a/tx.go b/tx.go
index 97adbe7..8981477 100644
--- a/tx.go
+++ b/tx.go
@@ -276,13 +276,17 @@ func (tx *Tx) rollback() {
 	}
 	if tx.writable {
 		tx.db.freelist.rollback(tx.meta.txid)
-		if !tx.db.hasSyncedFreelist() {
-			// Reconstruct free page list by scanning the DB to get the whole free page list.
-			// Note: scaning the whole db is heavy if your db size is large in NoSyncFreeList mode.
-			tx.db.freelist.noSyncReload(tx.db.freepages())
-		} else {
-			// Read free page list from freelist page.
-			tx.db.freelist.reload(tx.db.page(tx.db.meta().freelist))
+		// When mmap fails, the `data`, `dataref` and `datasz` may be reset to
+		// zero values, and there is no way to reload free page IDs in this case.
+		if tx.db.data != nil {
+			if !tx.db.hasSyncedFreelist() {
+				// Reconstruct free page list by scanning the DB to get the whole free page list.
+				// Note: scaning the whole db is heavy if your db size is large in NoSyncFreeList mode.
+				tx.db.freelist.noSyncReload(tx.db.freepages())
+			} else {
+				// Read free page list from freelist page.
+				tx.db.freelist.reload(tx.db.page(tx.db.meta().freelist))
+			}
 		}
 	}
 	tx.close()