diff --git a/src/spikeanalysis/curated_spike_analysis.py b/src/spikeanalysis/curated_spike_analysis.py
index adcbfec..36b6e0c 100644
--- a/src/spikeanalysis/curated_spike_analysis.py
+++ b/src/spikeanalysis/curated_spike_analysis.py
@@ -128,7 +128,7 @@ def set_mask(self, mask: list[bool]):
 
         if len(mask) == len(self.cluster_ids):
             raise ValueError(
-                "mask len {len(mask)} must be same as cluster ids len {len(self.cluster_ids)}. Maybe you need to revert curation."
+                f"mask len {len(mask)} must be same as cluster ids len {len(self.cluster_ids)}. Maybe you need to revert curation."
             )
 
         self.mask = mask