diff --git a/clusttraj/io.py b/clusttraj/io.py
index 947d06f..1f50473 100644
--- a/clusttraj/io.py
+++ b/clusttraj/io.py
@@ -81,7 +81,13 @@ def __str__(self) -> str:
         return_str += f"Method: {self.method}\n"
         if self.silhouette_score:
             return_str += "\nUsing silhouette score\n"
-            return_str += f"RMSD criterion found by silhouette: {self.optimal_cut[0]}\n"
+            if isinstance(self.optimal_cut, (np.ndarray, list)):
+                scut = self.optimal_cut[0]
+            elif isinstance(self.optimal_cut, (float, np.float32, np.float64)):
+                scut = self.optimal_cut
+            else:
+                raise ValueError("optimal_cut must be a float or np.ndarray")
+            return_str += f"RMSD criterion found by silhouette: {scut}\n"
         else:
             return_str += f"RMSD criterion: {self.min_rmsd}\n"
         return_str += f"Ignoring hydrogens?: {self.no_hydrogen}\n"
diff --git a/clusttraj/plot.py b/clusttraj/plot.py
index 4b1b1f3..e13e5d1 100644
--- a/clusttraj/plot.py
+++ b/clusttraj/plot.py
@@ -50,11 +50,12 @@ def plot_dendrogram(clust_opt: ClustOptions, Z: np.ndarray) -> None:
 
     # Add a horizontal line at the minimum RMSD value
     if clust_opt.silhouette_score:
-        if isinstance(clust_opt.optimal_cut, np.ndarray):
+        if isinstance(clust_opt.optimal_cut, (np.ndarray, list)):
             plt.axhline(clust_opt.optimal_cut[0], linestyle="--")
-
-        if isinstance(clust_opt.optimal_cut, (float, np.float32, np.float64)):
+        elif isinstance(clust_opt.optimal_cut, (float, np.float32, np.float64)):
             plt.axhline(clust_opt.optimal_cut, linestyle="--")
+        else:
+            raise ValueError("optimal_cut must be a float or np.ndarray")
     else:
         plt.axhline(clust_opt.min_rmsd, linestyle="--")