diff --git a/libguile/struct.c b/libguile/struct.c
index 8d7b00161..a7a719a4f 100644
--- a/libguile/struct.c
+++ b/libguile/struct.c
@@ -144,6 +144,8 @@ set_vtable_access_fields (SCM vtable)
                                    scm_ash (SCM_INUM1,
                                             scm_from_size_t (field)));
 
+  scm_gc_pin_object (SCM_I_CURRENT_THREAD, unboxed_fields);
+
   /* Record computed size of vtable's instances.  */
   SCM_SET_VTABLE_FLAGS (vtable, 0);
   SCM_STRUCT_DATA_SET (vtable, scm_vtable_index_size, nfields);