diff --git a/gc.c b/gc.c
index 6281cf4a..445d6383 100644
--- a/gc.c
+++ b/gc.c
@@ -99,7 +99,7 @@ SEXP_API sexp_uint_t sexp_allocated_bytes (sexp ctx, sexp x) {
   res = sexp_type_size_of_object(t, x) + SEXP_GC_PAD;
 #if SEXP_USE_DEBUG_GC
   if (res == 0) {
-    fprintf(stderr, SEXP_BANNER("%p zero-size object: %p"), ctx, x);
+    fprintf(stderr, SEXP_BANNER("%p zero-size object: %p (type tag: %d)"), ctx, x, sexp_pointer_tag(x));
     return 1;
   }
 #endif