diff --git a/streaming/base/format/mds/encodings.py b/streaming/base/format/mds/encodings.py
index c28d0058d..8223e8f60 100644
--- a/streaming/base/format/mds/encodings.py
+++ b/streaming/base/format/mds/encodings.py
@@ -258,7 +258,7 @@ def encode(self, obj: npt.NDArray) -> bytes:
             parts.append(part)
         else:
             if obj.shape != self.shape:
-                raise ValueError('Wrong shape: expected {self.shape}, got {obj.shape}.')
+                raise ValueError(f'Wrong shape: expected {self.shape}, got {obj.shape}.')
 
         # Encode the array values.
         part = obj.tobytes()