diff --git a/test/custom.mjs b/test/custom.mjs
index a5eece7..d232dd5 100644
--- a/test/custom.mjs
+++ b/test/custom.mjs
@@ -136,5 +136,10 @@ describe('A custom instance', () => {
       // sake of catching new specializations). Not sure whether that will be
       // OK or a problem that will have to be dealt with.
       assert.strictEqual(inst.typeMerge(3, 3n), 'Merge to Complex')
+      // But types that truly cannot be merged should throw a TypeError
+      // Should add a variation of this with a more usual type once there is
+      // one not interconvertible with others...
+      inst.install(genericSubtract)
+      assert.throws(() => inst.typeMerge(3, undefined), TypeError)
    })
 })