diff --git a/src/c/instance.d b/src/c/instance.d
index 17334f66d..6a07d1e71 100644
--- a/src/c/instance.d
+++ b/src/c/instance.d
@@ -254,7 +254,8 @@ enum ecl_built_in_classes {
 	ECL_BUILTIN_READTABLE,
 	ECL_BUILTIN_CODE_BLOCK,
 	ECL_BUILTIN_FOREIGN_DATA,
-	ECL_BUILTIN_FRAME
+	ECL_BUILTIN_FRAME,
+	ECL_BUILTIN_WEAK_POINTER
 #ifdef ECL_THREADS
         ,
 	ECL_BUILTIN_PROCESS,
@@ -356,6 +357,8 @@ cl_class_of(cl_object x)
 		index = ECL_BUILTIN_FOREIGN_DATA; break;
 	case t_frame:
 		index = ECL_BUILTIN_FRAME; break;
+	case t_weak_pointer:
+		index = ECL_BUILTIN_WEAK_POINTER; break;
 	default:
 		ecl_internal_error("not a lisp data object");
 	}