diff --git a/changelog.d/8705.misc b/changelog.d/8705.misc
new file mode 100644
index 000000000..1189464a0
--- /dev/null
+++ b/changelog.d/8705.misc
@@ -0,0 +1 @@
+Improve the error returned when a non-string displayname or avatar_url is used when updating a user's profile.
\ No newline at end of file
diff --git a/synapse/handlers/profile.py b/synapse/handlers/profile.py
index 14348faaf..74a1ddd78 100644
--- a/synapse/handlers/profile.py
+++ b/synapse/handlers/profile.py
@@ -189,7 +189,9 @@ class ProfileHandler(BaseHandler):
                 )
 
         if not isinstance(new_displayname, str):
-            raise SynapseError(400, "Invalid displayname")
+            raise SynapseError(
+                400, "'displayname' must be a string", errcode=Codes.INVALID_PARAM
+            )
 
         if len(new_displayname) > MAX_DISPLAYNAME_LEN:
             raise SynapseError(
@@ -273,7 +275,9 @@ class ProfileHandler(BaseHandler):
                 )
 
         if not isinstance(new_avatar_url, str):
-            raise SynapseError(400, "Invalid displayname")
+            raise SynapseError(
+                400, "'avatar_url' must be a string", errcode=Codes.INVALID_PARAM
+            )
 
         if len(new_avatar_url) > MAX_AVATAR_URL_LEN:
             raise SynapseError(