diff --git a/libs/pbd/sndfile_manager.cc b/libs/pbd/sndfile_manager.cc
index c6a52416ed..5f08e634d6 100644
--- a/libs/pbd/sndfile_manager.cc
+++ b/libs/pbd/sndfile_manager.cc
@@ -69,6 +69,7 @@ SndFileDescriptor::close ()
 {
 	/* we must have a lock on the FileManager's mutex */
 
+	assert (_sndfile);
 	sf_close (_sndfile);
 	_sndfile = 0;
 }