diff --git a/xugrid/ugrid/ugridbase.py b/xugrid/ugrid/ugridbase.py
index 3c7e2321a..ac28841bf 100644
--- a/xugrid/ugrid/ugridbase.py
+++ b/xugrid/ugrid/ugridbase.py
@@ -215,7 +215,7 @@ def _single_topology(dataset: xr.Dataset):
             raise ValueError("Dataset contains no UGRID topology variable.")
         elif n_topology > 1:
             raise ValueError(
-                "Dataset contains {n_topology} topology variables, "
+                f"Dataset contains {n_topology} topology variables, "
                 "please specify the topology variable name to use."
             )
         return topologies[0]