diff --git a/hamilflow/models/harmonic_oscillator.py b/hamilflow/models/harmonic_oscillator.py
index 973f2e7..9f29d3a 100644
--- a/hamilflow/models/harmonic_oscillator.py
+++ b/hamilflow/models/harmonic_oscillator.py
@@ -312,7 +312,7 @@ def _x(self, t: "Sequence[float] | npt.ArrayLike") -> npt.ArrayLike:
             x = self._x_critical_damped(t)
         else:
             raise ValueError(
-                "System type is not damped harmonic oscillator: {self.system.type}",
+                f"System type is not damped harmonic oscillator: {self.system.type}",
             )
 
         return x