diff --git a/icefall/diagnostics.py b/icefall/diagnostics.py
index 2e6087ad5..609e25626 100644
--- a/icefall/diagnostics.py
+++ b/icefall/diagnostics.py
@@ -130,6 +130,8 @@ class TensorDiagnostic(object):
             x = x[0]
         if not isinstance(x, Tensor):
             return
+        if x.numel() == 0:  # for empty tensor
+            return
         x = x.detach().clone()
         if x.ndim == 0:
             x = x.unsqueeze(0)