diff --git a/fluent-bundle/src/resolver.ts b/fluent-bundle/src/resolver.ts
index 5d13ef95..dd8afc35 100644
--- a/fluent-bundle/src/resolver.ts
+++ b/fluent-bundle/src/resolver.ts
@@ -154,15 +154,12 @@ function resolveVariableReference(
   let arg: FluentVariable;
   if (scope.params) {
     // We're inside a TermReference. It's OK to reference undefined parameters.
-    if (Object.prototype.hasOwnProperty.call(scope.params, name)) {
+    if (name in scope.params) {
       arg = scope.params[name];
     } else {
       return new FluentNone(`$${name}`);
     }
-  } else if (
-    scope.args &&
-    Object.prototype.hasOwnProperty.call(scope.args, name)
-  ) {
+  } else if (scope.args && name in scope.args) {
     // We're in the top-level Pattern or inside a MessageReference. Missing
     // variables references produce ReferenceErrors.
     arg = scope.args[name];
diff --git a/fluent-bundle/test/object_prototype_test.js b/fluent-bundle/test/object_prototype_test.js
index c44eb0ae..913c6508 100644
--- a/fluent-bundle/test/object_prototype_test.js
+++ b/fluent-bundle/test/object_prototype_test.js
@@ -26,7 +26,7 @@ suite("Interesting Object properties", function () {
       const val = bundle.formatPattern(msg.value, {}, errs);
       assert.strictEqual(val, "{$constructor}");
       assert.strictEqual(errs.length, 1);
-      assert(errs[0] instanceof ReferenceError); // unknown variable
+      assert(errs[0] instanceof TypeError); // variable type not supported
     });
 
     test("empty args with null prototype", function () {
@@ -60,7 +60,7 @@ suite("Interesting Object properties", function () {
       const val = bundle.formatPattern(msg.value, {}, errs);
       assert.strictEqual(val, "{$hasOwnProperty}");
       assert.strictEqual(errs.length, 1);
-      assert(errs[0] instanceof ReferenceError); // unknown variable
+      assert(errs[0] instanceof TypeError); // variable type not supported
     });
 
     test("empty args with null prototype", function () {
@@ -94,7 +94,7 @@ suite("Interesting Object properties", function () {
       const val = bundle.formatPattern(msg.value, {}, errs);
       assert.strictEqual(val, "{$isPrototypeOf}");
       assert.strictEqual(errs.length, 1);
-      assert(errs[0] instanceof ReferenceError); // unknown variable
+      assert(errs[0] instanceof TypeError); // variable type not supported
     });
 
     test("empty args with null prototype", function () {
@@ -128,7 +128,7 @@ suite("Interesting Object properties", function () {
       const val = bundle.formatPattern(msg.value, {}, errs);
       assert.strictEqual(val, "{$propertyIsEnumerable}");
       assert.strictEqual(errs.length, 1);
-      assert(errs[0] instanceof ReferenceError); // unknown variable
+      assert(errs[0] instanceof TypeError); // variable type not supported
     });
 
     test("empty args with null prototype", function () {
@@ -166,7 +166,7 @@ suite("Interesting Object properties", function () {
       const val = bundle.formatPattern(msg.value, {}, errs);
       assert.strictEqual(val, "{$toLocaleString}");
       assert.strictEqual(errs.length, 1);
-      assert(errs[0] instanceof ReferenceError); // unknown variable
+      assert(errs[0] instanceof TypeError); // variable type not supported
     });
 
     test("empty args with null prototype", function () {
@@ -200,7 +200,7 @@ suite("Interesting Object properties", function () {
       const val = bundle.formatPattern(msg.value, {}, errs);
       assert.strictEqual(val, "{$toString}");
       assert.strictEqual(errs.length, 1);
-      assert(errs[0] instanceof ReferenceError); // unknown variable
+      assert(errs[0] instanceof TypeError); // variable type not supported
     });
 
     test("empty args with null prototype", function () {
@@ -234,7 +234,7 @@ suite("Interesting Object properties", function () {
       const val = bundle.formatPattern(msg.value, {}, errs);
       assert.strictEqual(val, "{$valueOf}");
       assert.strictEqual(errs.length, 1);
-      assert(errs[0] instanceof ReferenceError); // unknown variable
+      assert(errs[0] instanceof TypeError); // variable type not supported
     });
 
     test("empty args with null property", function () {