diff --git a/libguile/struct.c b/libguile/struct.c
index 2d36303b4..ad88ac844 100644
--- a/libguile/struct.c
+++ b/libguile/struct.c
@@ -364,7 +364,7 @@ scm_free_structs (void *dummy1 SCM_UNUSED,
 	{
 	  SCM vtable = SCM_STRUCT_VTABLE (chain);
 	  if (SCM_STRUCT_GC_CHAIN (vtable) != 0 && vtable != chain)
-	    SCM_SET_GC_MARK (vtable);
+	    SCM_SET_STRUCT_MARK (vtable);
 	  chain = SCM_STRUCT_GC_CHAIN (chain);
 	}
       /* Free unmarked structs.  */
@@ -374,9 +374,9 @@ scm_free_structs (void *dummy1 SCM_UNUSED,
 	{
 	  SCM obj = chain;
 	  chain = SCM_STRUCT_GC_CHAIN (chain);
-	  if (SCM_GC_MARK_P (obj))
+	  if (SCM_STRUCT_MARK_P (obj))
 	    {
-	      SCM_CLEAR_GC_MARK (obj);
+	      SCM_CLEAR_STRUCT_MARK (obj);
 	      SCM_SET_STRUCT_GC_CHAIN (obj, newchain);
 	      newchain = obj;
 	    }
@@ -897,8 +897,8 @@ scm_struct_prehistory ()
 {
   scm_i_structs_to_free = SCM_EOL;
   scm_c_hook_add (&scm_before_sweep_c_hook, scm_struct_gc_init, 0, 0);
-  /* With the new lazy sweep GC, the point at which the entire heap is
-     swept is just before the mark phase. */
+  /* With lazy sweep GC, the point at which the entire heap is swept
+     is just before the mark phase. */
   scm_c_hook_add (&scm_before_mark_c_hook, scm_free_structs, 0, 0);
 }
 
diff --git a/libguile/struct.h b/libguile/struct.h
index f00a8d844..10c0d65c3 100644
--- a/libguile/struct.h
+++ b/libguile/struct.h
@@ -79,6 +79,15 @@ SCM_API SCM scm_struct_table;
 
 #define SCM_STRUCT_GC_CHAIN(X) SCM_CELL_OBJECT_3 (X)
 #define SCM_SET_STRUCT_GC_CHAIN(X, Y) SCM_SET_CELL_OBJECT_3 (X, Y)
+
+/* For clearing structs. We can't use the regular GC mark bits, as
+   meddling with them at random times would mess up the invariants of
+   the garbage collector.
+ */
+#define SCM_STRUCT_MARK_P(X) SCM_CELL_WORD_2 (X)
+#define SCM_SET_STRUCT_MARK(X) SCM_SET_CELL_WORD_2 (X, 0x1)
+#define SCM_CLEAR_STRUCT_MARK(X) SCM_SET_CELL_WORD_2 (X, 0x0)
+
 SCM_INTERNAL SCM scm_i_structs_to_free;