diff --git a/libvips/iofuncs/cache.c b/libvips/iofuncs/cache.c
index d8db2e3a..9129b152 100644
--- a/libvips/iofuncs/cache.c
+++ b/libvips/iofuncs/cache.c
@@ -445,16 +445,22 @@ vips_operation_equal( VipsOperation *a, VipsOperation *b )
 	return( FALSE );
 }
 
+void *
+vips__cache_once_init( void )
+{
+	vips_cache_lock = vips_g_mutex_new();
+
+	vips_cache_table = g_hash_table_new( 
+		(GHashFunc) vips_operation_hash, 
+		(GEqualFunc) vips_operation_equal );
+}
+
 void
 vips__cache_init( void )
 {
-	if( !vips_cache_table ) {
-		vips_cache_lock = vips_g_mutex_new();
+	static GOnce once = G_ONCE_INIT;
 
-		vips_cache_table = g_hash_table_new( 
-			(GHashFunc) vips_operation_hash, 
-			(GEqualFunc) vips_operation_equal );
-	}
+	g_once( &once, (GThreadFunc) vips__cache_once_init, NULL );
 }
 
 static void *
diff --git a/libvips/resample/thumbnail.c b/libvips/resample/thumbnail.c
index 53d35a11..672c7b1a 100644
--- a/libvips/resample/thumbnail.c
+++ b/libvips/resample/thumbnail.c
@@ -281,8 +281,7 @@ vips_thumbnail_open( VipsThumbnail *thumbnail )
 	}
 	else if( vips_isprefix( "VipsForeignLoadPdf", thumbnail->loader ) ||
 		vips_isprefix( "VipsForeignLoadSvg", thumbnail->loader ) ) {
-		scale = 1.0 / 
-			vips_thumbnail_calculate_common_shrink( thumbnail, 
+		scale = 1.0 / vips_thumbnail_calculate_common_shrink( thumbnail, 
 			thumbnail->input_width, thumbnail->input_height );
 
 		g_info( "loading PDF/SVG with factor %g pre-scale", scale ); 
@@ -698,6 +697,11 @@ vips_thumbnail_file_open( VipsThumbnail *thumbnail, int shrink, double scale )
 {
 	VipsThumbnailFile *file = (VipsThumbnailFile *) thumbnail;
 
+	/* If both shrink and scale have been set, something is wrong. It
+	 * should be one or the other (or neither).
+	 */
+	g_assert( shrink == 1 || scale == 1.0 );
+
 	if( shrink != 1 ) 
 		return( vips_image_new_from_file( file->filename, 
 			"access", VIPS_ACCESS_SEQUENTIAL,