diff --git a/src/allmydata/dirnode.py b/src/allmydata/dirnode.py
index 7b6052919..1af655574 100644
--- a/src/allmydata/dirnode.py
+++ b/src/allmydata/dirnode.py
@@ -205,8 +205,8 @@ class Adder(object):
         return new_contents
 
 def _encrypt_rw_uri(writekey, rw_uri):
-    precondition(isinstance(rw_uri, str), rw_uri)
-    precondition(isinstance(writekey, str), writekey)
+    precondition(isinstance(rw_uri, bytes), rw_uri)
+    precondition(isinstance(writekey, bytes), writekey)
 
     salt = hashutil.mutable_rwcap_salt_hash(rw_uri)
     key = hashutil.mutable_rwcap_key_hash(salt, writekey)
@@ -264,26 +264,26 @@ def _pack_normalized_children(children, writekey, deep_immutable=False):
             assert isinstance(metadata, dict)
             rw_uri = child.get_write_uri()
             if rw_uri is None:
-                rw_uri = ""
-            assert isinstance(rw_uri, str), rw_uri
+                rw_uri = b""
+            assert isinstance(rw_uri, bytes), rw_uri
 
             # should be prevented by MustBeDeepImmutableError check above
             assert not (rw_uri and deep_immutable)
 
             ro_uri = child.get_readonly_uri()
             if ro_uri is None:
-                ro_uri = ""
-            assert isinstance(ro_uri, str), ro_uri
+                ro_uri = b""
+            assert isinstance(ro_uri, bytes), ro_uri
             if writekey is not None:
                 writecap = netstring(_encrypt_rw_uri(writekey, rw_uri))
             else:
                 writecap = ZERO_LEN_NETSTR
-            entry = "".join([netstring(name.encode("utf-8")),
+            entry = b"".join([netstring(name.encode("utf-8")),
                              netstring(strip_prefix_for_ro(ro_uri, deep_immutable)),
                              writecap,
-                             netstring(json.dumps(metadata))])
+                             netstring(json.dumps(metadata).encode("utf-8"))])
         entries.append(netstring(entry))
-    return "".join(entries)
+    return b"".join(entries)
 
 @implementer(IDirectoryNode, ICheckable, IDeepCheckable)
 class DirectoryNode(object):
@@ -352,7 +352,7 @@ class DirectoryNode(object):
         # cleartext. The 'name' is UTF-8 encoded, and should be normalized to NFC.
         # The rwcapdata is formatted as:
         # pack("16ss32s", iv, AES(H(writekey+iv), plaintext_rw_uri), mac)
-        assert isinstance(data, str), (repr(data), type(data))
+        assert isinstance(data, bytes), (repr(data), type(data))
         # an empty directory is serialized as an empty string
         if data == "":
             return AuxValueDict()
@@ -555,8 +555,8 @@ class DirectoryNode(object):
         return d
 
     def set_uri(self, namex, writecap, readcap, metadata=None, overwrite=True):
-        precondition(isinstance(writecap, (str,type(None))), writecap)
-        precondition(isinstance(readcap, (str,type(None))), readcap)
+        precondition(isinstance(writecap, (bytes, type(None))), writecap)
+        precondition(isinstance(readcap, (bytes, type(None))), readcap)
 
         # We now allow packing unknown nodes, provided they are valid
         # for this type of directory.
@@ -577,8 +577,8 @@ class DirectoryNode(object):
             else:
                 assert len(e) == 3
                 writecap, readcap, metadata = e
-            precondition(isinstance(writecap, (str,type(None))), writecap)
-            precondition(isinstance(readcap, (str,type(None))), readcap)
+            precondition(isinstance(writecap, (bytes,type(None))), writecap)
+            precondition(isinstance(readcap, (bytes,type(None))), readcap)
 
             # We now allow packing unknown nodes, provided they are valid
             # for this type of directory.
diff --git a/src/allmydata/test/test_system.py b/src/allmydata/test/test_system.py
index c151bb09e..19adeff20 100644
--- a/src/allmydata/test/test_system.py
+++ b/src/allmydata/test/test_system.py
@@ -1687,7 +1687,7 @@ class SystemTest(SystemTestMixin, RunBinTahoeMixin, unittest.TestCase):
             d1.addCallback(self.log, "publish finished")
             def _stash_uri(filenode):
                 self.uri = filenode.get_uri()
-                assert isinstance(self.uri, str), (self.uri, filenode)
+                assert isinstance(self.uri, bytes), (self.uri, filenode)
             d1.addCallback(_stash_uri)
             return d1
         d.addCallback(_made_subdir1)
@@ -2174,7 +2174,7 @@ class SystemTest(SystemTestMixin, RunBinTahoeMixin, unittest.TestCase):
                 filename = os.path.join(dirpath, filenames[0])
                 # peek at the magic to see if it is a chk share
                 magic = open(filename, "rb").read(4)
-                if magic == '\x00\x00\x00\x01':
+                if magic == b'\x00\x00\x00\x01':
                     break
         else:
             self.fail("unable to find any uri_extension files in %r"