diff --git a/tidy3d/plugins/dispersion/fit_fast.py b/tidy3d/plugins/dispersion/fit_fast.py
index 71e99bda2..2bbe49a7c 100644
--- a/tidy3d/plugins/dispersion/fit_fast.py
+++ b/tidy3d/plugins/dispersion/fit_fast.py
@@ -149,10 +149,12 @@ class FastFitterData(AdvancedFastFitterParam):
         title="eps_inf",
         description="Value of ``eps_inf``.",
     )
-    poles: ArrayComplex1D = Field(
+    poles: Optional[ArrayComplex1D] = Field(
         None, title="Pole frequencies in eV", description="Pole frequencies in eV"
     )
-    residues: ArrayComplex1D = Field(None, title="Residues in eV", description="Residues in eV")
+    residues: Optional[ArrayComplex1D] = Field(
+        None, title="Residues in eV", description="Residues in eV"
+    )
 
     passivity_optimized: Optional[bool] = Field(
         False,
@@ -188,7 +190,11 @@ def _generate_initial_poles(cls, val, values):
         """Generate initial poles."""
         if val is not None:
             return val
-        if values["logspacing"] is None or values["smooth"] is None:
+        if (
+            values.get("logspacing") is None
+            or values.get("smooth") is None
+            or values.get("num_poles") is None
+        ):
             return None
         omega = values["omega"]
         num_poles = values["num_poles"]
@@ -211,7 +217,7 @@ def _generate_initial_residues(cls, val, values):
         """Generate initial residues."""
         if val is not None:
             return val
-        poles = values["poles"]
+        poles = values.get("poles")
         if poles is None:
             return None
         return np.zeros(len(poles))
@@ -675,6 +681,11 @@ def fit(
             Best fitting result: (dispersive medium, weighted RMS error).
         """
 
+        if max_num_poles < min_num_poles:
+            raise ValidationError(
+                "Dispersion fitter cannot have 'max_num_poles' less than 'min_num_poles'."
+            )
+
         omega = PoleResidue.angular_freq_to_eV(PoleResidue.Hz_to_angular_freq(self.freqs[::-1]))
         eps = self.eps_data[::-1]