diff --git a/panda/src/grutil/geoMipTerrain.cxx b/panda/src/grutil/geoMipTerrain.cxx
index c34a50dcc2..3aad8e814a 100644
--- a/panda/src/grutil/geoMipTerrain.cxx
+++ b/panda/src/grutil/geoMipTerrain.cxx
@@ -70,7 +70,7 @@ generate_block(unsigned short mx,
   // Create vertex data and writers
   PT(GeomVertexData) vdata = new GeomVertexData(_root.get_name(),
                    GeomVertexFormat::register_format(format), Geom::UH_stream);
-  vdata->set_num_rows((_block_size + 1) * (_block_size + 1));
+  vdata->unclean_set_num_rows((_block_size + 1) * (_block_size + 1));
   GeomVertexWriter cwriter;
   if (_has_color_map) {
     cwriter=GeomVertexWriter(vdata, "color"  );