diff --git a/libvips/foreign/openslide2vips.c b/libvips/foreign/openslide2vips.c
index 3eaf405f..24262d98 100644
--- a/libvips/foreign/openslide2vips.c
+++ b/libvips/foreign/openslide2vips.c
@@ -279,7 +279,7 @@ vips__openslide_generate( VipsRegion *out,
 	 * TILE_HEIGHT pixels and on a tile boundary.
 	 */
 	g_assert( (r->left % TILE_WIDTH) == 0 );
-	g_assert( (r->height % TILE_HEIGHT) == 0 );
+	g_assert( (r->top % TILE_HEIGHT) == 0 );
 	g_assert( r->width <= TILE_WIDTH );
 	g_assert( r->height <= TILE_HEIGHT );