Index: src/library/methods/R/RMethodUtils.R
===================================================================
--- src/library/methods/R/RMethodUtils.R	(revision 76003)
+++ src/library/methods/R/RMethodUtils.R	(working copy)
@@ -830,6 +830,14 @@
                identical(cldef@package, pkg)) {
                 .uncacheClass(cl, cldef)
                 .removeSuperclassBackRefs(cl, cldef, searchWhere)
+                # Adapted from `removeClass` we need to de-register references
+                # to union superclasses from the package being detached
+                if(is(cldef, 'ClassUnionRepresentation') &&
+                   length(cldef@subclasses)) {
+                    subclasses <- names(cldef@subclasses)
+                    for(what in subclasses)
+                        .removeSuperClass(what, cl)
+                }
             }
         }
     }