diff --git a/xformers/ops/memory_efficient_attention.py b/xformers/ops/memory_efficient_attention.py
index 6694c89de4..5476e858cd 100644
--- a/xformers/ops/memory_efficient_attention.py
+++ b/xformers/ops/memory_efficient_attention.py
@@ -270,6 +270,8 @@ def forward_no_grad(
         attn_bias: Optional[Union[torch.Tensor, AttentionMask]],
         p: float,
     ) -> torch.Tensor:
+        if attn_bias is not None and not isinstance(attn_bias, LowerTriangularMask):
+            raise NotImplementedError("Unsupported attn_bias type")
         return cls.FORWARD_OPERATOR(
             query=query,
             key=key,
@@ -283,6 +285,8 @@ def forward_no_grad(
 
     @classmethod
     def forward(cls, ctx, query, key, value, attn_bias, p):
+        if attn_bias is not None and not isinstance(attn_bias, LowerTriangularMask):
+            raise NotImplementedError("Unsupported attn_bias type")
         causal = isinstance(attn_bias, LowerTriangularMask)
         out, lse = cls.FORWARD_OPERATOR(
             query=query,
@@ -438,6 +442,8 @@ def prepare_inputs(
 
     @classmethod
     def forward(cls, ctx, query, key, value, attn_bias, p):
+        if attn_bias is not None and not isinstance(attn_bias, LowerTriangularMask):
+            raise NotImplementedError("Unsupported attn_bias type")
         causal = isinstance(attn_bias, LowerTriangularMask)
         return_softmax = False
         ctx_flash = ctx if ctx is not None else SimpleNamespace()