diff --git a/piker/data/__init__.py b/piker/data/__init__.py
index c2b06d83..f40f3c52 100644
--- a/piker/data/__init__.py
+++ b/piker/data/__init__.py
@@ -186,7 +186,7 @@ async def allocate_persistent_feed(
     )
 
     # do history validation?
-    assert opened, "Persistent shm for sym was already open?!"
+    assert opened, f'Persistent shm for {symbol} was already open?!'
     # if not opened:
     #     raise RuntimeError("Persistent shm for sym was already open?!")