diff --git a/csharp/ql/src/Language Abuse/UselessCastToSelf.ql b/csharp/ql/src/Language Abuse/UselessCastToSelf.ql
index d1a08e1db1c5..96f93131fa6a 100644
--- a/csharp/ql/src/Language Abuse/UselessCastToSelf.ql	
+++ b/csharp/ql/src/Language Abuse/UselessCastToSelf.ql	
@@ -19,5 +19,6 @@ where
   type = cast.getTargetType() and
   type = e.getType() and
   not type instanceof NullType and
+  not type instanceof UnknownType and
   not e.(ImplicitDelegateCreation).getArgument() instanceof AnonymousFunctionExpr
 select cast, "This cast is redundant because the expression already has type " + type + "."
diff --git a/csharp/ql/src/change-notes/2026-08-20-harden-useless-cast-to-self.md b/csharp/ql/src/change-notes/2026-08-20-harden-useless-cast-to-self.md
new file mode 100644
index 000000000000..bb5bb85b86e7
--- /dev/null
+++ b/csharp/ql/src/change-notes/2026-08-20-harden-useless-cast-to-self.md
@@ -0,0 +1,4 @@
+---
+category: fix
+---
+* The query `cs/useless-cast-to-self` no longer reports casts when both the expression type and the cast target type are unknown, which can occur in `build-mode: none` databases.