diff --git a/sam3/train/loss/loss_fns.py b/sam3/train/loss/loss_fns.py
index 8fa17741e..aa4ed7cea 100644
--- a/sam3/train/loss/loss_fns.py
+++ b/sam3/train/loss/loss_fns.py
@@ -149,6 +149,10 @@ def sigmoid_focal_loss(
     """
     if not (0 <= alpha <= 1) and triton:
         raise RuntimeError(f"Alpha should be in [0,1], got {alpha}")
+    # The Triton backward path is numerically invalid for gamma == 0 because it
+    # computes (1 - p_t) ** (gamma - 1), i.e. an inverse power at zero.
+    if triton and float(gamma) == 0.0:
+        triton = False
     if triton:
         if reduce and not loss_on_multimask:
             loss = triton_sigmoid_focal_loss_reduce(inputs, targets, alpha, gamma)