diff --git a/src/bsp_level.cc b/src/bsp_level.cc
index 57819d1a..e64750e5 100644
--- a/src/bsp_level.cc
+++ b/src/bsp_level.cc
@@ -1545,7 +1545,7 @@ static void PutZVertices(ZLibContext &zcontext)
 	{
 		raw_v2_vertex_t raw;
 
-		vertex_t *vert = lev_vertices[i];
+		const vertex_t *vert = lev_vertices[i];
 
 		if (! vert->is_new)
 			continue;