diff --git a/models.py b/models.py
index aac4772..bc93c7f 100644
--- a/models.py
+++ b/models.py
@@ -800,6 +800,7 @@ class WindowAttention(nn.Module):
             x: input features with shape of (num_windows*B, N, C)
             mask: (0/-inf) mask with shape of (num_windows, Wh*Ww, Wh*Ww) or None
         """
+        B_, N, C = x.shape
         qkv_bias = None
         if self.q_bias is not None:
             qkv_bias = torch.cat((self.q_bias, torch.zeros_like(self.v_bias, requires_grad=False), self.v_bias))