diff --git a/src/spine/backbones/base.py b/src/spine/backbones/base.py
index 5a78b03..add0ad5 100644
--- a/src/spine/backbones/base.py
+++ b/src/spine/backbones/base.py
@@ -13,11 +13,16 @@
 
 @dataclass
 class EncodedEvent:
-    """What every backbone returns."""
+    """What every backbone returns.
+
+    ``cls`` is optional: a backbone with no event-level readout leaves it
+    ``None``, and consumers that need an event embedding (the query encoder)
+    must guard for it. Per-token pretexts read ``tokens`` and ignore ``cls``.
+    """
 
     tokens: Tensor  # [B, L, D] per-pulse token embeddings
     token_mask: Tensor  # [B, L] bool, True = real pulse (not padding)
-    cls: Tensor  # [B, D] pooled event embedding
+    cls: Tensor | None = None  # [B, D] pooled event embedding, or None
 
 
 class Backbone(nn.Module):
diff --git a/src/spine/pretrain/curtain/head.py b/src/spine/pretrain/curtain/head.py
index ac8f2ad..bf31a7c 100644
--- a/src/spine/pretrain/curtain/head.py
+++ b/src/spine/pretrain/curtain/head.py
@@ -84,7 +84,16 @@ def forward(self, query_pos: Tensor, enc: EncodedEvent) -> Tensor:
 
         Returns:
             [B, Q, D] per-query embeddings.
+
+        Raises:
+            ValueError: If the backbone returned no event-level embedding
+                (``enc.cls is None``); this encoder requires one.
         """
+        if enc.cls is None:
+            raise ValueError(
+                "QueryCrossAttnEncoder requires an event-level `cls` embedding, "
+                "but the backbone returned cls=None"
+            )
         kv = torch.cat([enc.cls.unsqueeze(1), enc.tokens], dim=1)  # [B,1+L,D]
         ones = torch.ones(
             enc.token_mask.shape[0], 1, dtype=torch.bool, device=enc.token_mask.device