diff --git a/src/depth_pro/depth_pro.py b/src/depth_pro/depth_pro.py
index f31b4e16..0cec10ed 100644
--- a/src/depth_pro/depth_pro.py
+++ b/src/depth_pro/depth_pro.py
@@ -280,6 +280,11 @@ def infer(
 
         canonical_inverse_depth, fov_deg = self.forward(x)
         if f_px is None:
+            if fov_deg is None:
+                raise ValueError(
+                    "f_px must be provided when use_fov_head=False, "
+                    "as the model cannot estimate focal length without the FOV head."
+                )
             f_px = 0.5 * W / torch.tan(0.5 * torch.deg2rad(fov_deg.to(torch.float)))
         
         inverse_depth = canonical_inverse_depth * (W / f_px)