diff --git a/synapse/crypto/event_signing.py b/synapse/crypto/event_signing.py
index d01ac5075c..1dfa727fcf 100644
--- a/synapse/crypto/event_signing.py
+++ b/synapse/crypto/event_signing.py
@@ -131,12 +131,12 @@ def compute_event_signature(event_dict, signature_name, signing_key):
     return redact_json["signatures"]
 
 
-def add_hashes_and_signatures(event, signature_name, signing_key,
+def add_hashes_and_signatures(event_dict, signature_name, signing_key,
                               hash_algorithm=hashlib.sha256):
     """Add content hash and sign the event
 
     Args:
-        event_dict (EventBuilder): The event to add hashes to and sign
+        event_dict (dict): The event to add hashes to and sign
         signature_name (str): The name of the entity signing the event
             (typically the server's hostname).
         signing_key (syutil.crypto.SigningKey): The key to sign with
@@ -144,16 +144,12 @@ def add_hashes_and_signatures(event, signature_name, signing_key,
             to hash the event
     """
 
-    name, digest = compute_content_hash(
-        event.get_pdu_json(), hash_algorithm=hash_algorithm,
-    )
+    name, digest = compute_content_hash(event_dict, hash_algorithm=hash_algorithm)
 
-    if not hasattr(event, "hashes"):
-        event.hashes = {}
-    event.hashes[name] = encode_base64(digest)
+    event_dict.setdefault("hashes", {})[name] = encode_base64(digest)
 
-    event.signatures = compute_event_signature(
-        event.get_pdu_json(),
+    event_dict["signatures"] = compute_event_signature(
+        event_dict,
         signature_name=signature_name,
         signing_key=signing_key,
     )
diff --git a/tests/crypto/test_event_signing.py b/tests/crypto/test_event_signing.py
index b2536c1e69..71aa731439 100644
--- a/tests/crypto/test_event_signing.py
+++ b/tests/crypto/test_event_signing.py
@@ -18,7 +18,7 @@ import nacl.signing
 from unpaddedbase64 import decode_base64
 
 from synapse.crypto.event_signing import add_hashes_and_signatures
-from synapse.events.builder import EventBuilder
+from synapse.events import FrozenEvent
 
 from tests import unittest
 
@@ -40,20 +40,18 @@ class EventSigningTestCase(unittest.TestCase):
         self.signing_key.version = KEY_VER
 
     def test_sign_minimal(self):
-        builder = EventBuilder(
-            {
-                'event_id': "$0:domain",
-                'origin': "domain",
-                'origin_server_ts': 1000000,
-                'signatures': {},
-                'type': "X",
-                'unsigned': {'age_ts': 1000000},
-            }
-        )
+        event_dict = {
+            'event_id': "$0:domain",
+            'origin': "domain",
+            'origin_server_ts': 1000000,
+            'signatures': {},
+            'type': "X",
+            'unsigned': {'age_ts': 1000000},
+        }
 
-        add_hashes_and_signatures(builder, HOSTNAME, self.signing_key)
+        add_hashes_and_signatures(event_dict, HOSTNAME, self.signing_key)
 
-        event = builder.build()
+        event = FrozenEvent(event_dict)
 
         self.assertTrue(hasattr(event, 'hashes'))
         self.assertIn('sha256', event.hashes)
@@ -71,23 +69,21 @@ class EventSigningTestCase(unittest.TestCase):
         )
 
     def test_sign_message(self):
-        builder = EventBuilder(
-            {
-                'content': {'body': "Here is the message content"},
-                'event_id': "$0:domain",
-                'origin': "domain",
-                'origin_server_ts': 1000000,
-                'type': "m.room.message",
-                'room_id': "!r:domain",
-                'sender': "@u:domain",
-                'signatures': {},
-                'unsigned': {'age_ts': 1000000},
-            }
-        )
+        event_dict = {
+            'content': {'body': "Here is the message content"},
+            'event_id': "$0:domain",
+            'origin': "domain",
+            'origin_server_ts': 1000000,
+            'type': "m.room.message",
+            'room_id': "!r:domain",
+            'sender': "@u:domain",
+            'signatures': {},
+            'unsigned': {'age_ts': 1000000},
+        }
 
-        add_hashes_and_signatures(builder, HOSTNAME, self.signing_key)
+        add_hashes_and_signatures(event_dict, HOSTNAME, self.signing_key)
 
-        event = builder.build()
+        event = FrozenEvent(event_dict)
 
         self.assertTrue(hasattr(event, 'hashes'))
         self.assertIn('sha256', event.hashes)