diff --git a/src/lexer.ts b/src/lexer.ts
index 462badb..f7100d1 100644
--- a/src/lexer.ts
+++ b/src/lexer.ts
@@ -1000,12 +1000,15 @@ export class Lexer {
     if (ctx === LexContext.CommandStart) {
       if (!hasExpansions && !quoted) {
         const fc = text.charCodeAt(0);
-        if (
-          ((fc >= CH_a && fc <= CH_z && text.length <= 8) || fc === CH_BANG || fc === CH_LBRACE || fc === CH_RBRACE) &&
-          text in RESERVED_WORDS
-        ) {
-          setToken(out, RESERVED_WORDS[text], text, tokenStart, wordEnd);
-          return;
+        if ((fc >= CH_a && fc <= CH_z && text.length <= 8) || fc === CH_BANG || fc === CH_LBRACE || fc === CH_RBRACE) {
+          // Single lookup, then a typeof check — `text in RESERVED_WORDS` would also match
+          // inherited Object.prototype members, making `toString` and `valueOf` parse as
+          // keywords. Own reserved words are always numeric Token values.
+          const reserved = RESERVED_WORDS[text];
+          if (typeof reserved === "number") {
+            setToken(out, reserved, text, tokenStart, wordEnd);
+            return;
+          }
         }
         if (fc === CH_LBRACKET && text === "[[") {
           setToken(out, Token.DblLBracket, text, tokenStart, wordEnd);
diff --git a/test/parser.test.ts b/test/parser.test.ts
index 36a9b1f..733be2f 100644
--- a/test/parser.test.ts
+++ b/test/parser.test.ts
@@ -51,6 +51,23 @@ test("set and trap commands", () => {
   assert.equal(ast.commands.length, 4);
 });
 
+// Command names that collide with Object.prototype members must not be mistaken
+// for reserved words by the keyword lookup.
+test("Object.prototype member names are ordinary command names", () => {
+  for (const name of ["toString", "valueOf", "constructor", "hasOwnProperty", "__proto__", "isPrototypeOf"]) {
+    const ast = parse(`${name} arg`);
+    assert.equal(ast.commands.length, 1, `no command for: ${name}`);
+    assert.equal(getCmd(ast).name?.text, name);
+    assert.deepEqual(args(getCmd(ast)), ["arg"]);
+  }
+});
+
+test("Object.prototype member name does not truncate the rest of the script", () => {
+  const ast = parse("echo hi; toString foo; echo bye");
+  assert.equal(ast.commands.length, 3);
+  assert.equal(getCmd(ast, 2).name?.text, "echo");
+});
+
 // ── Real-world patterns ─────────────────────────────────────────────
 
 test("real-world scripts parse without errors", () => {