diff --git a/libmproxy/protocol/http.py b/libmproxy/protocol/http.py
index a05f3597e..212bec964 100644
--- a/libmproxy/protocol/http.py
+++ b/libmproxy/protocol/http.py
@@ -59,6 +59,8 @@ class _StreamingHttpLayer(_HttpLayer):
         raise NotImplementedError()
 
     def send_response(self, response):
+        if response.body == CONTENT_MISSING:
+            raise HttpError(502, "Cannot assemble flow with CONTENT_MISSING")
         self.send_response_headers(response)
         self.send_response_body(response, response.body)