diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index f12c2775f21..5196df9db4f 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,3 +1,8 @@
+2004-10-11  Mark Mitchell  <mark@codesourcery.com>
+
+	PR c++/17929
+	* decl2.c (finish_anon_union): Robustify.
+
 2004-10-11  Nathan Sidwell  <nathan@codesourcery.com>
 
 	* cp-tree.h (get_dynamic_cast_base_type): Rename to ...
diff --git a/gcc/cp/decl2.c b/gcc/cp/decl2.c
index 896fdfc1834..7a55907ab6e 100644
--- a/gcc/cp/decl2.c
+++ b/gcc/cp/decl2.c
@@ -1182,9 +1182,15 @@ build_anon_union_vars (tree object)
 void
 finish_anon_union (tree anon_union_decl)
 {
-  tree type = TREE_TYPE (anon_union_decl);
+  tree type;
   tree main_decl;
-  bool public_p = TREE_PUBLIC (anon_union_decl);
+  bool public_p;
+
+  if (anon_union_decl == error_mark_node)
+    return;
+
+  type = TREE_TYPE (anon_union_decl);
+  public_p = TREE_PUBLIC (anon_union_decl);
 
   /* The VAR_DECL's context is the same as the TYPE's context.  */
   DECL_CONTEXT (anon_union_decl) = DECL_CONTEXT (TYPE_NAME (type));