diff --git a/changelog.d/8517.bugfix b/changelog.d/8517.bugfix
new file mode 100644
index 000000000..1ab623c59
--- /dev/null
+++ b/changelog.d/8517.bugfix
@@ -0,0 +1 @@
+Fix error code for `/profile/{userId}/displayname` to be `M_BAD_JSON`.
diff --git a/synapse/rest/client/v1/profile.py b/synapse/rest/client/v1/profile.py
index b686cd671..e7fcd2b1f 100644
--- a/synapse/rest/client/v1/profile.py
+++ b/synapse/rest/client/v1/profile.py
@@ -59,7 +59,9 @@ class ProfileDisplaynameRestServlet(RestServlet):
         try:
             new_name = content["displayname"]
         except Exception:
-            return 400, "Unable to parse name"
+            raise SynapseError(
+                code=400, msg="Unable to parse name", errcode=Codes.BAD_JSON,
+            )
 
         await self.profile_handler.set_displayname(user, requester, new_name, is_admin)