What is the recommended method for saving/loading a TxDb object?

> example(makeTxDb)

mkTxDb> transcripts <- data.frame(
mkTxDb+                    tx_id=1:3,
mkTxDb+                    tx_chrom="chr1",
mkTxDb+                    tx_strand=c("-", "+", "+"),
mkTxDb+                    tx_start=c(1, 2001, 2001),
mkTxDb+                    tx_end=c(999, 2199, 2199))

mkTxDb> splicings <-  data.frame(
mkTxDb+                    tx_id=c(1L, 2L, 2L, 2L, 3L, 3L),
mkTxDb+                    exon_rank=c(1, 1, 2, 3, 1, 2),
mkTxDb+                    exon_start=c(1, 2001, 2101, 2131, 2001, 2131),
mkTxDb+                    exon_end=c(999, 2085, 2144, 2199, 2085, 2199),
mkTxDb+                    cds_start=c(1, 2022, 2101, 2131, NA, NA),
mkTxDb+                    cds_end=c(999, 2085, 2144, 2193, NA, NA))

mkTxDb> txdb <- makeTxDb(transcripts, splicings)
Warning message:
In .normarg_makeTxDb_chrominfo(chrominfo, transcripts$tx_chrom,  :
  chromosome lengths and circularity flags are not available for this TxDb object

> txdb
TxDb object:
# Db type: TxDb
# Supporting package: GenomicFeatures
# transcript_nrow: 3
# exon_nrow: 4
# cds_nrow: 4
# Db created by: GenomicFeatures package from Bioconductor
# Creation time: 2015-06-09 07:13:11 -0700 (Tue, 09 Jun 2015)
# GenomicFeatures version at creation time: 1.20.1
# RSQLite version at creation time: 1.0.0
# DBSCHEMAVERSION: 1.1


> save(txdb, file = "tmp.Rdata")
> rm(txdb)
> load("tmp.Rdata")
> txdb
TxDb object:
Error in sqliteSendQuery(con, statement, bind.data) :
  expired SQLiteConnection

saveDb() / loadDb() in AnnotationDbi.