diff --git a/changelog.d/4460.bugfix b/changelog.d/4460.bugfix
new file mode 100644
index 0000000000..8c5d5b4e0e
--- /dev/null
+++ b/changelog.d/4460.bugfix
@@ -0,0 +1 @@
+Fix UnboundLocalError in post_urlencoded_get_json
diff --git a/synapse/http/client.py b/synapse/http/client.py
index afcf698b29..47a1f82ff0 100644
--- a/synapse/http/client.py
+++ b/synapse/http/client.py
@@ -333,9 +333,10 @@ class SimpleHttpClient(object):
             "POST", uri, headers=Headers(actual_headers), data=query_bytes
         )
 
+        body = yield make_deferred_yieldable(readBody(response))
+
         if 200 <= response.code < 300:
-            body = yield make_deferred_yieldable(treq.json_content(response))
-            defer.returnValue(body)
+            defer.returnValue(json.loads(body))
         else:
             raise HttpResponseException(response.code, response.phrase, body)