diff --git a/src/graph/graph.hh b/src/graph/graph.hh
index 64878a8..e4a5d40 100644
--- a/src/graph/graph.hh
+++ b/src/graph/graph.hh
@@ -70,8 +70,8 @@
       {
         DEBUG_MSG (SUBSET_REPACK, nullptr,
                    "vertex [%lu] bytes != [%lu] bytes, depth = %u",
-                   table_size (),
-                   other.table_size (),
+                   (unsigned long) table_size (),
+                   (unsigned long) other.table_size (),
                    depth);
 
         auto a = as_bytes ();