diff --git a/backend/plustek_pp.c b/backend/plustek_pp.c
index fcef98bfd..f48c3dc02 100644
--- a/backend/plustek_pp.c
+++ b/backend/plustek_pp.c
@@ -471,7 +471,7 @@ static SANE_Status do_cancel( Plustek_Scanner *scanner, SANE_Bool closepipe  )
 
 	scanner->scanning = SANE_FALSE;
 
-	if( scanner->reader_pid != -1 ) {
+	if( sanei_thread_is_valid( scanner->reader_pid )) {
 
 		DBG( _DBG_PROC, ">>>>>>>> killing reader_process <<<<<<<<\n" );
 
@@ -2014,7 +2014,7 @@ SANE_Status sane_start( SANE_Handle handle )
 	s->w_pipe     = fds[1];
 	s->reader_pid = sanei_thread_begin( reader_process, s );
 
-	if( s->reader_pid == -1 ) {
+	if(!sanei_thread_is_valid( s->reader_pid )) {
 		DBG( _DBG_ERROR, "ERROR: could not create child process\n" );
 		s->scanning = SANE_FALSE;
 		s->hw->close( s->hw );