diff --git a/changelog.d/15944.bugfix b/changelog.d/15944.bugfix
new file mode 100644
index 000000000000..9cbbad49071d
--- /dev/null
+++ b/changelog.d/15944.bugfix
@@ -0,0 +1 @@
+Add dic instance check before accessing auth session.
diff --git a/synapse/handlers/auth.py b/synapse/handlers/auth.py
index 2b0c50513095..dfab83690761 100644
--- a/synapse/handlers/auth.py
+++ b/synapse/handlers/auth.py
@@ -481,6 +481,11 @@ async def check_ui_auth(
 
         sid: Optional[str] = None
         authdict = clientdict.pop("auth", {})
+        if not isinstance(authdict, dict):
+            raise SynapseError(
+                400,
+                "Interactive auth not yet complete. Client data is not dictionary.",
+            )
         if "session" in authdict:
             sid = authdict["session"]