diff --git a/src/allmydata/dirnode.py b/src/allmydata/dirnode.py
index b06836f32..117c80e45 100644
--- a/src/allmydata/dirnode.py
+++ b/src/allmydata/dirnode.py
@@ -98,6 +98,9 @@ class Adder:
             if name in children:
                 if not self.overwrite:
                     raise ExistingChildError("child '%s' already exists" % name)
+
+                if self.overwrite == "only_files" and IDirectoryNode.providedBy(children[name][0]):
+                    raise ExistingChildError("child '%s' already exists" % name)
                 metadata = children[name][1].copy()
             else:
                 metadata = {"ctime": now,