diff --git a/libvips/iofuncs/source.c b/libvips/iofuncs/source.c
index 880c8bad..cc3764de 100644
--- a/libvips/iofuncs/source.c
+++ b/libvips/iofuncs/source.c
@@ -153,8 +153,7 @@ vips_source_test_features( VipsSource *source )
 	 * vips_source_seek() etc. or we might trigger seek emulation.
 	 */
 	if( source->data ||
-		(VIPS_CONNECTION( source )->descriptor != -1 &&
-			class->seek( source, 0, SEEK_CUR ) != -1) ) { 
+		class->seek( source, 0, SEEK_CUR ) != -1 ) { 
 		gint64 length;
 
 		VIPS_DEBUG_MSG( "    seekable source\n" );
@@ -349,16 +348,15 @@ vips_source_seek_real( VipsSource *source, gint64 offset, int whence )
 {
 	VipsConnection *connection = VIPS_CONNECTION( source );
 
-	gint64 new_pos;
-
 	VIPS_DEBUG_MSG( "vips_source_seek_real:\n" );
 
 	/* Like _read_real(), we must not set a vips_error. We need to use the
 	 * vips__seek() wrapper so we can seek long files on Windows.
 	 */
-	new_pos = vips__seek_no_error( connection->descriptor, offset, whence );
+	if( connection->descriptor != -1 )
+		return( vips__seek_no_error( connection->descriptor, offset, whence ) );
 
-	return( new_pos );
+	return( -1 );
 }
 
 static void