diff --git a/packages/compiler/src/frontend/lowering/coercions/state.ts b/packages/compiler/src/frontend/lowering/coercions/state.ts
index ad1d7c6ea..db9bb53ad 100644
--- a/packages/compiler/src/frontend/lowering/coercions/state.ts
+++ b/packages/compiler/src/frontend/lowering/coercions/state.ts
@@ -12,7 +12,10 @@ export class CoercionState {
   // Provenance for checked extraction; consumers must not infer it from names.
   readonly checkedNarrows = new Set<string>();
   // The extracted arm of each checked (non-defaulting) extraction helper.
-  readonly checkedNarrowArms = new Map<string, { unionId: string; tag: number; target: IrType }>();
+  readonly checkedNarrowArms = new Map<
+    string,
+    { unionId: string; tag: number; target: IrType; message?: string }
+  >();
   readonly retags = new Map<string, string>();
   readonly narrows = new Map<string, string>();
 }
diff --git a/packages/compiler/src/frontend/lowering/coercions/unions.ts b/packages/compiler/src/frontend/lowering/coercions/unions.ts
index 434e687a5..53c87e429 100644
--- a/packages/compiler/src/frontend/lowering/coercions/unions.ts
+++ b/packages/compiler/src/frontend/lowering/coercions/unions.ts
@@ -394,18 +394,21 @@ export function unionRetagHelper(
  * the catchable TypeError — divergence 38's lying-assertion stance (an
  * unchecked unionNarrow would misread the payload where JS lets the
  * impossible value flow on). Null when the target isn't a non-unit arm
- * of the union — those uses keep their erasure/fences. */
+ * of the union — those uses keep their erasure/fences. `message` replaces
+ * the failure message (the TypeError of the operation consuming the
+ * value, such as a call's "f is not a function"). */
 export function narrowedArmHelper(
   lowerer: Lowerer,
   fromId: string,
   target: IrType,
   loc: SrcLoc,
+  message?: string,
 ): string | null {
   const from = lowerer.unions.get(fromId);
   if (!from || isUnitType(target)) return null;
   const tag = lowerer.armTag(fromId, target);
   if (tag < 0) return null;
-  const key = `${fromId}:${tag}`;
+  const key = message === undefined ? `${fromId}:${tag}` : `${fromId}:${tag}:${message}`;
   const existing = lowerer.coercions.narrows.get(key);
   if (existing) {
     lowerer.coercions.checkedNarrows.add(existing);
@@ -414,9 +417,22 @@ export function narrowedArmHelper(
   const name = keyedHelperName(lowerer, "%union.narrow", key);
   lowerer.coercions.narrows.set(key, name);
   lowerer.coercions.checkedNarrows.add(name);
-  const fn = buildUnionNarrow(name, from, target, loc, (type) => lowerer.fmt(type));
+  const fn = buildUnionNarrow(
+    name,
+    from,
+    target,
+    loc,
+    (type) => lowerer.fmt(type),
+    null,
+    message ?? null,
+  );
   if (!fn) throw new InternalCompilerError("lowerer bug: invalid checked union extraction");
-  lowerer.coercions.checkedNarrowArms.set(name, { unionId: fromId, tag, target: from.arms[tag]! });
+  lowerer.coercions.checkedNarrowArms.set(name, {
+    unionId: fromId,
+    tag,
+    target: from.arms[tag]!,
+    ...(message === undefined ? {} : { message }),
+  });
   lowerer.liftedFns.push(fn);
   return name;
 }
@@ -458,7 +474,8 @@ export function checkedNarrowCall(
   let failure: IrExpr | null = null;
   for (let tag = from.arms.length - 1; tag >= 0; tag--) {
     if (tag === arm.tag) continue;
-    const message = unionNarrowFailureMessage(from.arms[tag]!, (t) => lowerer.fmt(t));
+    const message =
+      arm.message ?? unionNarrowFailureMessage(from.arms[tag]!, (t) => lowerer.fmt(t));
     const thrown = nodeThrowExpr(1, "", message, type, loc);
     failure =
       failure === null
diff --git a/packages/compiler/src/frontend/lowering/expressions/narrowing.ts b/packages/compiler/src/frontend/lowering/expressions/narrowing.ts
index 01cab9901..9f0559672 100644
--- a/packages/compiler/src/frontend/lowering/expressions/narrowing.ts
+++ b/packages/compiler/src/frontend/lowering/expressions/narrowing.ts
@@ -875,8 +875,15 @@ function checkedStaleArm(
 
 /** V8's spelling of a callee in "is not a function": a string-literal
  * element key reads as a property name (`o["a"]` is `o.a`), other keys
- * keep their brackets. */
+ * keep their brackets, parentheses and non-null assertions disappear, and
+ * a call result reads `f(...)`. */
 export function v8CalleeText(node: ts.Node): string {
+  // `!` is type-only syntax: the running JavaScript has no trace of it.
+  if (ts.isNonNullExpression(node) || ts.isParenthesizedExpression(node))
+    return v8CalleeText(node.expression);
+  // A call result prints with its arguments elided.
+  if (ts.isCallExpression(node) && !node.questionDotToken)
+    return `${v8CalleeText(node.expression)}(...)`;
   if (ts.isElementAccessExpression(node)) {
     const key = node.argumentExpression;
     const object = v8CalleeText(node.expression);
diff --git a/packages/compiler/src/frontend/lowering/lower-exprs.ts b/packages/compiler/src/frontend/lowering/lower-exprs.ts
index 20d05f3b7..006975085 100644
--- a/packages/compiler/src/frontend/lowering/lower-exprs.ts
+++ b/packages/compiler/src/frontend/lowering/lower-exprs.ts
@@ -89,7 +89,7 @@ import { classSymbolKeyOf } from "./symbol-fields.js";
 import { lowerTypeofExpr } from "./expressions/typeof.js";
 import { lowerIdentifierExpr } from "./expressions/identifiers.js";
 import { lowerFieldCompoundValue, lowerIncDec } from "./expressions/updates.js";
-import { lowerTernary } from "./expressions/narrowing.js";
+import { lowerTernary, v8CalleeText } from "./expressions/narrowing.js";
 import {
   classFieldTarget,
   lowerPropertyAccessExpr,
@@ -433,7 +433,12 @@ function lowerExprInner(lowerer: Lowerer, expr: ts.Expression): IrExpr {
       const operand = assertionOperand(lowerer, expr, inner, target ?? null, loc);
       if (operand) return operand;
       if (target && target.kind !== "union" && !typeEquals(target, inner.type)) {
-        const helper = lowerer.narrowedArmHelper(inner.type.unionId, target, loc);
+        const helper = lowerer.narrowedArmHelper(
+          inner.type.unionId,
+          target,
+          loc,
+          assertedCalleeMessage(expr, target),
+        );
         if (helper) {
           return lowerer.checkedNarrowCall(helper, inner, target, loc);
         }
@@ -1160,6 +1165,19 @@ const NUMERIC_COMPOUNDS = new Set<ts.SyntaxKind>([
   ts.SyntaxKind.GreaterThanGreaterThanGreaterThanEqualsToken,
 ]);
 
+/** The TypeError Node raises when an asserted callee (`t[k]!(…)`, `o.f!()`)
+ * holds no function: `!` is erased, so the call itself throws "<callee> is
+ * not a function" in V8's spelling of the callee. */
+function assertedCalleeMessage(expr: ts.NonNullExpression, target: IrType): string | undefined {
+  if (target.kind !== "func") return undefined;
+  let child: ts.Expression = expr;
+  while (ts.isParenthesizedExpression(child.parent)) child = child.parent;
+  const call = child.parent;
+  if (!ts.isCallExpression(call) || call.expression !== child || call.questionDotToken)
+    return undefined;
+  return `${v8CalleeText(expr.expression)} is not a function`;
+}
+
 /** Whether the use of `t[i]!` tells undefined from NaN: a converting use
  * that takes the value itself (see convertingUseOf), or a destination that
  * holds a missing element (see assertedElementKeepsUndefined) other than a
diff --git a/packages/compiler/src/frontend/lowering/lowerer.ts b/packages/compiler/src/frontend/lowering/lowerer.ts
index e49fde6bb..56e39ff0c 100644
--- a/packages/compiler/src/frontend/lowering/lowerer.ts
+++ b/packages/compiler/src/frontend/lowering/lowerer.ts
@@ -10525,8 +10525,8 @@ export class Lowerer {
     return unionRetagHelper(this, fromId, toId, loc, trappable);
   }
 
-  narrowedArmHelper(fromId: string, target: IrType, loc: SrcLoc): string | null {
-    return narrowedArmHelper(this, fromId, target, loc);
+  narrowedArmHelper(fromId: string, target: IrType, loc: SrcLoc, message?: string): string | null {
+    return narrowedArmHelper(this, fromId, target, loc, message);
   }
 
   checkedNarrowCall(helper: string, value: IrExpr, type: IrType, loc: SrcLoc): IrExpr {
diff --git a/packages/compiler/src/frontend/lowering/union-narrow.test.ts b/packages/compiler/src/frontend/lowering/union-narrow.test.ts
index ed60fac8d..5c2a81b5e 100644
--- a/packages/compiler/src/frontend/lowering/union-narrow.test.ts
+++ b/packages/compiler/src/frontend/lowering/union-narrow.test.ts
@@ -97,3 +97,22 @@ test("a deferred read requires a real undefined tag", () => {
   const value: IrExpr = { kind: "boolLit", value: false, type: BOOL, loc };
   expect(buildUnionNarrow("bad", from, BOOL, loc, () => "", value)).toBeNull();
 });
+
+test("a failure message replaces every other tag's TypeError", () => {
+  const fn = buildUnionNarrow(
+    "call",
+    union,
+    STRING,
+    loc,
+    (type) => type.kind,
+    null,
+    "f is not a function",
+  )!;
+  const thrown = branches(fn).flatMap((branch) => {
+    const statement = branch.then[0]!;
+    return statement.kind === "throw" ? [statement.value] : [];
+  });
+  expect(thrown).toHaveLength(4);
+  for (const value of thrown)
+    expect(value).toMatchObject({ kind: "libCall", args: [{ value: "f is not a function" }] });
+});
diff --git a/packages/compiler/src/frontend/lowering/union-narrow.ts b/packages/compiler/src/frontend/lowering/union-narrow.ts
index 3bb09cb26..a167631fc 100644
--- a/packages/compiler/src/frontend/lowering/union-narrow.ts
+++ b/packages/compiler/src/frontend/lowering/union-narrow.ts
@@ -20,7 +20,10 @@ export function unionNarrowFailureMessage(
  *
  * Deferred scalar fields can supply the established undefined default:
  * false for booleans or NaN for numbers. No other tag receives that default,
- * and reference-bearing targets always use checked extraction. */
+ * and reference-bearing targets always use checked extraction.
+ *
+ * `failureMessage` replaces the TypeError message of every other tag (the
+ * error Node raises for the operation consuming the value). */
 export function buildUnionNarrow(
   name: string,
   from: IrUnionDef,
@@ -28,6 +31,7 @@ export function buildUnionNarrow(
   loc: SrcLoc,
   formatType: (type: IrType) => string,
   undefinedDefault: IrExpr | null = null,
+  failureMessage: string | null = null,
 ): IrFunction | null {
   if (isUnitType(target) || target.kind === "void") return null;
   const targetTag = from.arms.findIndex((arm) => typeEquals(arm, target));
@@ -66,7 +70,7 @@ export function buildUnionNarrow(
             args: [
               {
                 kind: "strLit",
-                value: unionNarrowFailureMessage(arm, formatType),
+                value: failureMessage ?? unionNarrowFailureMessage(arm, formatType),
                 type: STRING,
                 loc,
               },
diff --git a/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/8240-asserted-callee-errors.ts.json b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/8240-asserted-callee-errors.ts.json
new file mode 100644
index 000000000..365bf7076
--- /dev/null
+++ b/packages/compiler/test/ts7/baselines/order-parity/tests/corpus/8240-asserted-callee-errors.ts.json
@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/8240-asserted-callee-errors.ts"
+ ],
+ "diags": []
+}
diff --git a/tests/corpus/8240-asserted-callee-errors.ts b/tests/corpus/8240-asserted-callee-errors.ts
new file mode 100644
index 000000000..91537ed70
--- /dev/null
+++ b/tests/corpus/8240-asserted-callee-errors.ts
@@ -0,0 +1,25 @@
+// A non-null assertion is erased in JavaScript, so calling an asserted callee
+// that holds undefined throws the call's own TypeError, with V8's spelling of
+// the callee: element keys, literal keys as names, parentheses and asserted
+// links dropped, call results as f(...).
+type F = (x: number) => number;
+const table: Record<string, F> = { inc: (x) => x + 1 };
+const obj: { cb?: F; inner?: { cb?: F } } = { inner: {} };
+const arr: (F | undefined)[] = [undefined];
+let maybe: F | undefined;
+function get(): F | undefined { return undefined; }
+const key = process.argv.length > 99 ? "inc" : "missing";
+const cases: [string, () => number][] = [
+  ["computed", () => table[key]!(1)],
+  ["literal key", () => table["missing"]!(1)],
+  ["dotted", () => obj.cb!(1)],
+  ["nested", () => obj.inner!.cb!(1)],
+  ["element", () => arr[0]!(1)],
+  ["local", () => maybe!(1)],
+  ["parens", () => (table[key])!(1)],
+  ["call result", () => get()!(1)],
+  ["parens outside", () => (table[key]!)(1)],
+];
+for (const [name, run] of cases) {
+  try { console.log(name, run()); } catch (e) { console.log(name, (e as Error).name + ": " + (e as Error).message); }
+}
diff --git a/tests/harness/corpus-costs/8240-asserted-callee-errors.ts.json b/tests/harness/corpus-costs/8240-asserted-callee-errors.ts.json
new file mode 100644
index 000000000..8c0e66c3c
--- /dev/null
+++ b/tests/harness/corpus-costs/8240-asserted-callee-errors.ts.json
@@ -0,0 +1 @@
+{ "plain": 3000, "san": 3600 }