diff --git a/lib/parser.js b/lib/parser.js
index 7fa2fc58..cb71c68e 100644
--- a/lib/parser.js
+++ b/lib/parser.js
@@ -102,7 +102,7 @@
     };
 
     Parser.prototype.assignOrPush = function(obj, key, newValue) {
-      if (!(key in obj)) {
+      if (!Object.prototype.hasOwnProperty.call(obj, key)) {
         if (!this.options.explicitArray) {
           return defineProperty(obj, key, newValue);
         } else {
diff --git a/src/parser.coffee b/src/parser.coffee
index dade8f39..9eda8f30 100644
--- a/src/parser.coffee
+++ b/src/parser.coffee
@@ -61,7 +61,7 @@ class exports.Parser extends events
         @emit err
 
   assignOrPush: (obj, key, newValue) =>
-    if key not of obj
+    if not Object::hasOwnProperty.call obj, key
       if not @options.explicitArray
         defineProperty obj, key, newValue
       else
diff --git a/test/parser.test.coffee b/test/parser.test.coffee
index f2758759..af132a4e 100644
--- a/test/parser.test.coffee
+++ b/test/parser.test.coffee
@@ -647,3 +647,23 @@ module.exports =
     .catch (err) ->
       assert.notEqual err, null
       test.finish()
+
+  # Regression test for issue #719: prototype chain property confusion
+  # Tags named after Object.prototype methods should parse correctly
+  'test prototype property names do not cause errors': skeleton({__xmlString: '<root><toString>value1</toString><valueOf>value2</valueOf><constructor>value3</constructor><hasOwnProperty>value4</hasOwnProperty></root>'}, (r) ->
+    console.log 'Result object: ' + util.inspect r, false, 10
+    # Verify all prototype-named tags parsed as strings, not functions
+    equ r.root.toString[0], 'value1'
+    equ r.root.valueOf[0], 'value2'
+    equ r.root.constructor[0], 'value3'
+    equ r.root.hasOwnProperty[0], 'value4'
+    # Verify no inherited functions leaked into the result
+    equ typeof r.root.toString[0], 'string'
+    equ typeof r.root.valueOf[0], 'string')
+
+  'test multiple prototype property tags parse correctly': skeleton({__xmlString: '<root><toString>a</toString><toString>b</toString></root>'}, (r) ->
+    console.log 'Result object: ' + util.inspect r, false, 10
+    # Multiple tags with prototype names should be collected into arrays
+    equ r.root.toString.length, 2
+    equ r.root.toString[0], 'a'
+    equ r.root.toString[1], 'b')