diff --git a/tiled/client/decoders.py b/tiled/client/decoders.py
index 74772de42..37002ca6f 100644
--- a/tiled/client/decoders.py
+++ b/tiled/client/decoders.py
@@ -14,6 +14,7 @@ def __init__(self):
 
         def decode(self, data: bytes) -> bytes:
             self._data.append(data)
+            return b""
 
         def flush(self) -> bytes:
             # Hide this here to defer the numpy import that it triggers.