diff --git a/synapse/api/events/__init__.py b/synapse/api/events/__init__.py
index 910c990b33..d68629209d 100644
--- a/synapse/api/events/__init__.py
+++ b/synapse/api/events/__init__.py
@@ -59,7 +59,7 @@ class SynapseEvent(JsonEncodedObject):
         "age_ts",
         "prev_content",
         "prev_state",
-        "pruned",
+        "pruned_because",
     ]
 
     internal_keys = [
diff --git a/synapse/storage/__init__.py b/synapse/storage/__init__.py
index 46d420b632..ac8ee89ab5 100644
--- a/synapse/storage/__init__.py
+++ b/synapse/storage/__init__.py
@@ -206,7 +206,7 @@ class DataStore(RoomMemberStore, RoomStore,
         unrec = {
             k: v
             for k, v in event.get_full_dict().items()
-            if k not in vals.keys() and k not in ["deleted", "pruned"]
+            if k not in vals.keys() and k not in ["deleted", "pruned_because"]
         }
         vals["unrecognized_keys"] = json.dumps(unrec)
 
diff --git a/synapse/storage/_base.py b/synapse/storage/_base.py
index 3aa610c85c..d64119a473 100644
--- a/synapse/storage/_base.py
+++ b/synapse/storage/_base.py
@@ -398,7 +398,7 @@ class SQLBaseStore(object):
 
                 if del_evs:
                     prune_event(ev)
-                    ev.pruned = del_evs[0]
+                    ev.pruned_because = del_evs[0]
 
         return events