diff --git a/src/allmydata/introducer/client.py b/src/allmydata/introducer/client.py
index 62642d0af..0e0a56442 100644
--- a/src/allmydata/introducer/client.py
+++ b/src/allmydata/introducer/client.py
@@ -198,7 +198,7 @@ class IntroducerClient(service.Service, Referenceable):
         ann_d.update(ann)
         return ann_d
 
-    def publish(self, service_name, ann, signing_key=None):
+    def publish(self, service_name, ann, signing_key):
         # we increment the seqnum every time we publish something new
         current_seqnum, current_nonce = self._sequencer()
 
diff --git a/src/allmydata/introducer/interfaces.py b/src/allmydata/introducer/interfaces.py
index 9f08f1943..24fd3945f 100644
--- a/src/allmydata/introducer/interfaces.py
+++ b/src/allmydata/introducer/interfaces.py
@@ -73,7 +73,7 @@ class IIntroducerClient(Interface):
     publish their services to the rest of the world, and I help them learn
     about services available on other nodes."""
 
-    def publish(service_name, ann, signing_key=None):
+    def publish(service_name, ann, signing_key):
         """Publish the given announcement dictionary (which must be
         JSON-serializable), plus some additional keys, to the world.
 
@@ -83,8 +83,7 @@ class IIntroducerClient(Interface):
         the signing_key, if present, otherwise it is derived from the
         'anonymous-storage-FURL' key.
 
-        If signing_key= is set to an instance of SigningKey, it will be
-        used to sign the announcement."""
+        signing_key (a SigningKey) will be used to sign the announcement."""
 
     def subscribe_to(service_name, callback, *args, **kwargs):
         """Call this if you will eventually want to use services with the