diff --git a/src/drv_imap.c b/src/drv_imap.c
index ae85524..9520cdd 100644
--- a/src/drv_imap.c
+++ b/src/drv_imap.c
@@ -817,7 +817,7 @@ static int
 parse_response_code( imap_store_t *ctx, struct imap_cmd_cb *cb, char *s )
 {
 	imap_t *imap = ctx->imap;
-	char *arg, *p;
+	char *arg, *earg, *p;
 
 	if (*s != '[')
 		return RESP_OK;		/* no response code */
@@ -829,7 +829,9 @@ parse_response_code( imap_store_t *ctx, struct imap_cmd_cb *cb, char *s )
 	*p++ = 0;
 	arg = next_arg( &s );
 	if (!strcmp( "UIDVALIDITY", arg )) {
-		if (!(arg = next_arg( &s )) || !(ctx->gen.uidvalidity = atoi( arg ))) {
+		if (!(arg = next_arg( &s )) ||
+		    (ctx->gen.uidvalidity = strtoll( arg, &earg, 10 ), *earg))
+		{
 			fprintf( stderr, "IMAP error: malformed UIDVALIDITY status\n" );
 			return RESP_BAD;
 		}
@@ -847,7 +849,8 @@ parse_response_code( imap_store_t *ctx, struct imap_cmd_cb *cb, char *s )
 		for (; isspace( (unsigned char)*p ); p++);
 		fprintf( stderr, "*** IMAP ALERT *** %s\n", p );
 	} else if (cb && cb->ctx && !strcmp( "APPENDUID", arg )) {
-		if (!(arg = next_arg( &s )) || !(ctx->gen.uidvalidity = atoi( arg )) ||
+		if (!(arg = next_arg( &s )) ||
+		    (ctx->gen.uidvalidity = strtoll( arg, &earg, 10 ), *earg) ||
 		    !(arg = next_arg( &s )) || !(*(int *)cb->ctx = atoi( arg )))
 		{
 			fprintf( stderr, "IMAP error: malformed APPENDUID status\n" );
diff --git a/src/sync.c b/src/sync.c
index b9bfdb3..59035b6 100644
--- a/src/sync.c
+++ b/src/sync.c
@@ -474,7 +474,7 @@ sync_boxes( store_t *mctx, const char *mname,
 
 	nmmsg = nsmsg = 0;
 
-	mctx->uidvalidity = sctx->uidvalidity = 0;
+	mctx->uidvalidity = sctx->uidvalidity = -1;
 	mopts = sopts = 0;
 	makeopts( chan->sops, chan->slave, &sopts, chan->master, &mopts );
 	makeopts( chan->mops, chan->master, &mopts, chan->slave, &sopts );
@@ -510,7 +510,8 @@ sync_boxes( store_t *mctx, const char *mname,
 	nfasprintf( &jname, "%s.journal", dname );
 	nfasprintf( &nname, "%s.new", dname );
 	nfasprintf( &lname, "%s.lock", dname );
-	muidval = suidval = smaxxuid = mmaxuid = smaxuid = 0;
+	muidval = suidval = -1;
+	smaxxuid = mmaxuid = smaxuid = 0;
 	memset( &lck, 0, sizeof(lck) );
 #if SEEK_SET != 0
 	lck.l_whence = SEEK_SET;
@@ -705,7 +706,7 @@ sync_boxes( store_t *mctx, const char *mname,
 	info( "%d messages, %d recent\n", sctx->count, sctx->recent );
 	dump_box( sctx );
 
-	if (suidval && suidval != sctx->uidvalidity) {
+	if (suidval >= 0 && suidval != sctx->uidvalidity) {
 		fprintf( stderr, "Error: UIDVALIDITY of slave changed\n" );
 		ret = SYNC_FAIL;
 		goto bail;
@@ -809,13 +810,13 @@ sync_boxes( store_t *mctx, const char *mname,
 	info( "%d messages, %d recent\n", mctx->count, mctx->recent );
 	dump_box( mctx );
 
-	if (muidval && muidval != mctx->uidvalidity) {
+	if (muidval >= 0 && muidval != mctx->uidvalidity) {
 		fprintf( stderr, "Error: UIDVALIDITY of master changed\n" );
 		ret = SYNC_FAIL;
 		goto finish;
 	}
 
-	if (!muidval || !suidval) {
+	if (muidval < 0 || suidval < 0) {
 		muidval = mctx->uidvalidity;
 		suidval = sctx->uidvalidity;
 		Fprintf( jfp, "| %d %d\n", muidval, suidval );