diff --git a/docs/rules/no-import-assign.md b/docs/rules/no-import-assign.md
index 78b228dd828..49595cda2f8 100644
--- a/docs/rules/no-import-assign.md
+++ b/docs/rules/no-import-assign.md
@@ -16,8 +16,10 @@ import * as mod_ns from "./mod.mjs"
 
 mod = 1          // ERROR: 'mod' is readonly.
 named = 2        // ERROR: 'named' is readonly.
-mod_ns.named = 3 // ERROR: the members of 'mod_ns' is readonly.
+mod_ns.named = 3 // ERROR: The members of 'mod_ns' are readonly.
 mod_ns = {}      // ERROR: 'mod_ns' is readonly.
+// Can't extend 'mod_ns'
+Object.assign(mod_ns, { foo: "foo" }) // ERROR: The members of 'mod_ns' are readonly.
 ```
 
 Examples of **correct** code for this rule: