.

wcfs/internal/xbtree/δbtail.go

@@ -278,7 +278,9 @@ func (δΒtail *ΔTail) Get(ctx context.Context, root *Tree, key Key, at zodb.Ti
 
 	// XXX -> index lastXXXOf(key) | linear scan ↓ looking for change <= at
 	δTtail := δΒtail.byRoot[root]
-	// XXX nil -> ?
+	if δTtail == nil {
+		panicf("δBtail: root<%s> not tracked", root.POid())
+	}
 
 	for i := len(δTtail.vδT)-1; i >= 0; i-- {
 		δT := δTtail.vδT[i]