diff --git a/vit_pytorch/vit.py b/vit_pytorch/vit.py
index 5b34a44a..bfcad3c0 100644
--- a/vit_pytorch/vit.py
+++ b/vit_pytorch/vit.py
@@ -48,19 +48,27 @@ def __init__(self, dim, heads = 8, dim_head = 64, dropout = 0.):
         ) if project_out else nn.Identity()
 
     def forward(self, x):
-        x = self.norm(x)
-
-        qkv = self.to_qkv(x).chunk(3, dim = -1)
-        q, k, v = map(lambda t: rearrange(t, 'b n (h d) -> b h n d', h = self.heads), qkv)
-
+        assert x.dim() == 3, "Input x must have three dimensions: (batch_size, sequence_length, embedding_dim)"
+ 
+        qkv = self.to_qkv(x)
+        q, k, v = self.rearrange(qkv).chunk(3, dim=-1)
+ 
         dots = torch.matmul(q, k.transpose(-1, -2)) * self.scale
-
+ 
+        if mask is not None:
+            mask = mask.unsqueeze(1).expand(dots.size(0), self.heads, dots.size(2), dots.size(3))
+            dots = dots.masked_fill(~mask, -torch.finfo(dots.dtype).max)
+ 
         attn = self.attend(dots)
         attn = self.dropout(attn)
-
+ 
         out = torch.matmul(attn, v)
-        out = rearrange(out, 'b h n d -> b n (h d)')
-        return self.to_out(out)
+ 
+        out = self.rearrange(out, 'b h n d -> b n (h d)', h=self.heads).contiguous()
+ 
+        out = self.to_out(out)
+ 
+        return out
 
 class Transformer(nn.Module):
     def __init__(self, dim, depth, heads, dim_head, mlp_dim, dropout = 0.):