diff --git a/src/primitive.test.ts b/src/primitive.test.ts
index 6b1c9f8..3c445fe 100644
--- a/src/primitive.test.ts
+++ b/src/primitive.test.ts
@@ -326,6 +326,21 @@ test("getMatcherW factory rejects patterns outside its contract", () => {
     factory({ x: () => 1 as const });
 });
 
+// ============================================================================
+// Dispatch — runtime behavior
+// ============================================================================
+
+test("getMatcher: an unhandled shape throws without a fallback", () => {
+    // Arrange — an open universe types its handler map as an index signature,
+    // so the type system cannot prove the runtime map is exhaustive.
+    const handlers: Record<string, () => number> = { a: () => 1 };
+    const matcher = getMatcher<string>()(handlers);
+
+    // Act / Assert
+    assert.equal(matcher("a"), 1);
+    assert.throws(() => matcher("b"), /Unhandled shape: b/);
+});
+
 // ============================================================================
 // Autocomplete — the language server is the oracle, not the type system
 // ============================================================================