diff --git a/dalle2_pytorch/dalle2_pytorch.py b/dalle2_pytorch/dalle2_pytorch.py
index fb6770d..681df5d 100644
--- a/dalle2_pytorch/dalle2_pytorch.py
+++ b/dalle2_pytorch/dalle2_pytorch.py
@@ -3,7 +3,7 @@ import torch.nn.functional as F
 from torch import nn, einsum
 
 from einops import rearrange
-from einops_exts import rearrange_many
+from einops_exts import rearrange_many, repeat_many
 
 # use x-clip
 
@@ -82,23 +82,31 @@ class Attention(nn.Module):
         self.norm = RMSNorm(dim)
         self.dropout = nn.Dropout(dropout)
 
+        self.null_kv = nn.Parameter(torch.randn(heads, 2, dim_head))
         self.to_qkv = nn.Linear(dim, inner_dim * 3, bias = False)
         self.to_out = nn.Linear(inner_dim, dim, bias = False)
 
     def forward(self, x, mask = None):
-        n, device = x.shape[1], x.device
+        b, n, device = x.shape[:2], x.device
 
         x = self.norm(x)
         qkv = self.to_qkv(x).chunk(3, dim = -1)
 
         q, k, v = rearrange_many(qkv, 'b n (h d) -> b h n d')
 
+        # add null key / value for classifier free guidance in prior net
+
+        nk, nv = repeat_many(self.null_kv.unbind(dim = -2), 'h d -> b h 1 d', b = b)
+        k = torch.cat((nk, k), dim = -2)
+        v = torch.cat((nv, v), dim = -2)
+
         q = q * self.scale
 
         sim = einsum('b h i d, b h j d -> b h i j')
         max_neg_value = -torch.finfo(sim.dtype).max
 
         if exists(mask):
+            mask = F.pad(mask, (1, 0), value = True)
             mask = rearrange(mask, 'b j -> b 1 1 j')
             sim = sim.masked_fill(~mask, max_neg_value)