diff --git a/ddp_train_nerf.py b/ddp_train_nerf.py
index a217fdc..8988318 100644
--- a/ddp_train_nerf.py
+++ b/ddp_train_nerf.py
@@ -49,7 +49,10 @@ def intersect_sphere(ray_o, ray_d):
     p = ray_o + d1.unsqueeze(-1) * ray_d
     # consider the case where the ray does not intersect the sphere
     ray_d_cos = 1. / torch.norm(ray_d, dim=-1)
-    d2 = torch.sqrt(1. - torch.sum(p * p, dim=-1)) * ray_d_cos
+    p_norm_sq = torch.sum(p * p, dim=-1)
+    if (p_norm_sq >= 1.).any():
+        raise Exception('Not all your cameras are bounded by the unit sphere; please make sure the cameras are normalized properly!')
+    d2 = torch.sqrt(1. - p_norm_sq) * ray_d_cos
 
     return d1 + d2