--- a/mercurial/merge.py
+++ b/mercurial/merge.py
@@ -384,13 +384,9 @@ def update(repo, node, branchmerge, forc
         if branchmerge:
             raise util.Abort(_("there is nothing to merge, just use "
                                "'hg update' or look at 'hg heads'"))
-    elif not branchmerge:
-        if not overwrite:
-            if wc.files():
-                raise util.Abort(_("outstanding uncommited changes, use "
-                                   "'hg update -C' to lose changes"))
-            else:
-                overwrite = True
+    elif not (overwrite or branchmerge):
+        raise util.Abort(_("update spans branches, use 'hg merge' "
+                           "or 'hg update -C' to lose changes"))
     if branchmerge and not forcemerge:
         if wc.modified() or wc.added() or wc.removed():
             raise util.Abort(_("outstanding uncommitted changes"))