diff --git a/faust/streams.py b/faust/streams.py
index 4a2a6b5a0..2e6e4cd91 100644
--- a/faust/streams.py
+++ b/faust/streams.py
@@ -207,7 +207,7 @@ def _iter_ll(self, dir_: _LinkedListDirection) -> Iterator[StreamT]:
         while node:
             if node in seen:
                 raise RuntimeError(
-                    'Loop in Stream.{dir_.attr}: Call support!')
+                    f'Loop in Stream.{dir_.attr}: Call support!')
             seen.add(node)
             yield node
             node = dir_.getter(node)