diff --git a/src/array.js b/src/array.js
index 6ed1476..df080a7 100644
--- a/src/array.js
+++ b/src/array.js
@@ -67,6 +67,25 @@
 }
 
 
+function SparseJoinWithSeparator(array, len, convert, separator) {
+  var keys = GetSortedArrayKeys(array, %GetArrayKeys(array, len));
+  var totalLength = 0;
+  var elements = new InternalArray(keys.length * 2);
+  var previousKey = -1;
+  for (var i = 0; i < keys.length; i++) {
+    var key = keys[i];
+    if (key != previousKey) {  // keys may contain duplicates.
+      var e = array[key];
+      if (!IS_STRING(e)) e = convert(e);
+      elements[i * 2] = key;
+      elements[i * 2 + 1] = e;
+      previousKey = key;
+    }
+  }
+  return %SparseJoinWithSeparator(elements, len, separator);
+}
+
+
 // Optimized for sparse arrays if separator is ''.
 function SparseJoin(array, len, convert) {
   var keys = GetSortedArrayKeys(array, %GetArrayKeys(array, len));
@@ -110,8 +129,12 @@
 
   // Attempt to convert the elements.
   try {
-    if (UseSparseVariant(array, length, is_array) && (separator.length == 0)) {
-      return SparseJoin(array, length, convert);
+    if (UseSparseVariant(array, length, is_array)) {
+      if (separator.length == 0) {
+        return SparseJoin(array, length, convert);
+      } else {
+        return SparseJoinWithSeparator(array, length, convert, separator);
+      }
     }
 
     // Fast case for one-element arrays.
@@ -129,10 +152,8 @@
       var elements_length = 0;
       for (var i = 0; i < length; i++) {
         var e = array[i];
-        if (!IS_UNDEFINED(e)) {
-          if (!IS_STRING(e)) e = convert(e);
-          elements[elements_length++] = e;
-        }
+        if (!IS_STRING(e)) e = convert(e);
+        elements[elements_length++] = e;
       }
       elements.length = elements_length;
       var result = %_FastAsciiArrayJoin(elements, '');
@@ -151,11 +172,12 @@
     } else {
       for (var i = 0; i < length; i++) {
         var e = array[i];
-        if (IS_NUMBER(e)) elements[i] = %_NumberToString(e);
-        else {
-          if (!IS_STRING(e)) e = convert(e);
+          if (IS_NUMBER(e)) {
+            e = %_NumberToString(e);
+          } else if (!IS_STRING(e)) {
+            e = convert(e);
+          }
           elements[i] = e;
-        }
       }
     }
     var result = %_FastAsciiArrayJoin(elements, separator);
@@ -375,6 +397,11 @@
 
 
 function ArrayJoin(separator) {
+  if (IS_NULL_OR_UNDEFINED(this) && !IS_UNDETECTABLE(this)) {
+    throw MakeTypeError("called_on_null_or_undefined",
+                        ["Array.prototype.join"]);
+  }
+
   if (IS_UNDEFINED(separator)) {
     separator = ',';
   } else if (!IS_STRING(separator)) {
@@ -391,6 +418,11 @@
 // Removes the last element from the array and returns it. See
 // ECMA-262, section 15.4.4.6.
 function ArrayPop() {
+  if (IS_NULL_OR_UNDEFINED(this) && !IS_UNDETECTABLE(this)) {
+    throw MakeTypeError("called_on_null_or_undefined",
+                        ["Array.prototype.pop"]);
+  }
+
   var n = TO_UINT32(this.length);
   if (n == 0) {
     this.length = n;
@@ -407,6 +439,11 @@
 // Appends the arguments to the end of the array and returns the new
 // length of the array. See ECMA-262, section 15.4.4.7.
 function ArrayPush() {
+  if (IS_NULL_OR_UNDEFINED(this) && !IS_UNDETECTABLE(this)) {
+    throw MakeTypeError("called_on_null_or_undefined",
+                        ["Array.prototype.push"]);
+  }
+
   var n = TO_UINT32(this.length);
   var m = %_ArgumentsLength();
   for (var i = 0; i < m; i++) {
@@ -418,6 +455,11 @@
 
 
 function ArrayConcat(arg1) {  // length == 1
+  if (IS_NULL_OR_UNDEFINED(this) && !IS_UNDETECTABLE(this)) {
+    throw MakeTypeError("called_on_null_or_undefined",
+                        ["Array.prototype.concat"]);
+  }
+
   var arg_count = %_ArgumentsLength();
   var arrays = new InternalArray(1 + arg_count);
   arrays[0] = this;
@@ -474,6 +516,11 @@
 
 
 function ArrayReverse() {
+  if (IS_NULL_OR_UNDEFINED(this) && !IS_UNDETECTABLE(this)) {
+    throw MakeTypeError("called_on_null_or_undefined",
+                        ["Array.prototype.reverse"]);
+  }
+
   var j = TO_UINT32(this.length) - 1;
 
   if (UseSparseVariant(this, j, IS_ARRAY(this))) {
@@ -505,6 +552,11 @@
 
 
 function ArrayShift() {
+  if (IS_NULL_OR_UNDEFINED(this) && !IS_UNDETECTABLE(this)) {
+    throw MakeTypeError("called_on_null_or_undefined",
+                        ["Array.prototype.shift"]);
+  }
+
   var len = TO_UINT32(this.length);
 
   if (len === 0) {
@@ -526,6 +578,11 @@
 
 
 function ArrayUnshift(arg1) {  // length == 1
+  if (IS_NULL_OR_UNDEFINED(this) && !IS_UNDETECTABLE(this)) {
+    throw MakeTypeError("called_on_null_or_undefined",
+                        ["Array.prototype.unshift"]);
+  }
+
   var len = TO_UINT32(this.length);
   var num_arguments = %_ArgumentsLength();
 
@@ -545,6 +602,11 @@
 
 
 function ArraySlice(start, end) {
+  if (IS_NULL_OR_UNDEFINED(this) && !IS_UNDETECTABLE(this)) {
+    throw MakeTypeError("called_on_null_or_undefined",
+                        ["Array.prototype.slice"]);
+  }
+
   var len = TO_UINT32(this.length);
   var start_i = TO_INTEGER(start);
   var end_i = len;
@@ -582,6 +644,11 @@
 
 
 function ArraySplice(start, delete_count) {
+  if (IS_NULL_OR_UNDEFINED(this) && !IS_UNDETECTABLE(this)) {
+    throw MakeTypeError("called_on_null_or_undefined",
+                        ["Array.prototype.splice"]);
+  }
+
   var num_arguments = %_ArgumentsLength();
 
   var len = TO_UINT32(this.length);
@@ -653,6 +720,11 @@
 
 
 function ArraySort(comparefn) {
+  if (IS_NULL_OR_UNDEFINED(this) && !IS_UNDETECTABLE(this)) {
+    throw MakeTypeError("called_on_null_or_undefined",
+                        ["Array.prototype.sort"]);
+  }
+
   // In-place QuickSort algorithm.
   // For short (length <= 22) arrays, insertion sort is used for efficiency.
 
@@ -914,6 +986,11 @@
 // preserving the semantics, since the calls to the receiver function can add
 // or delete elements from the array.
 function ArrayFilter(f, receiver) {
+  if (IS_NULL_OR_UNDEFINED(this) && !IS_UNDETECTABLE(this)) {
+    throw MakeTypeError("called_on_null_or_undefined",
+                        ["Array.prototype.filter"]);
+  }
+
   if (!IS_FUNCTION(f)) {
     throw MakeTypeError('called_non_callable', [ f ]);
   }
@@ -935,6 +1012,11 @@
 
 
 function ArrayForEach(f, receiver) {
+  if (IS_NULL_OR_UNDEFINED(this) && !IS_UNDETECTABLE(this)) {
+    throw MakeTypeError("called_on_null_or_undefined",
+                        ["Array.prototype.forEach"]);
+  }
+
   if (!IS_FUNCTION(f)) {
     throw MakeTypeError('called_non_callable', [ f ]);
   }
@@ -953,6 +1035,11 @@
 // Executes the function once for each element present in the
 // array until it finds one where callback returns true.
 function ArraySome(f, receiver) {
+  if (IS_NULL_OR_UNDEFINED(this) && !IS_UNDETECTABLE(this)) {
+    throw MakeTypeError("called_on_null_or_undefined",
+                        ["Array.prototype.some"]);
+  }
+
   if (!IS_FUNCTION(f)) {
     throw MakeTypeError('called_non_callable', [ f ]);
   }
@@ -970,6 +1057,11 @@
 
 
 function ArrayEvery(f, receiver) {
+  if (IS_NULL_OR_UNDEFINED(this) && !IS_UNDETECTABLE(this)) {
+    throw MakeTypeError("called_on_null_or_undefined",
+                        ["Array.prototype.every"]);
+  }
+
   if (!IS_FUNCTION(f)) {
     throw MakeTypeError('called_non_callable', [ f ]);
   }
@@ -986,6 +1078,11 @@
 }
 
 function ArrayMap(f, receiver) {
+  if (IS_NULL_OR_UNDEFINED(this) && !IS_UNDETECTABLE(this)) {
+    throw MakeTypeError("called_on_null_or_undefined",
+                        ["Array.prototype.map"]);
+  }
+
   if (!IS_FUNCTION(f)) {
     throw MakeTypeError('called_non_callable', [ f ]);
   }
@@ -1006,6 +1103,11 @@
 
 
 function ArrayIndexOf(element, index) {
+  if (IS_NULL_OR_UNDEFINED(this) && !IS_UNDETECTABLE(this)) {
+    throw MakeTypeError("called_on_null_or_undefined",
+                        ["Array.prototype.indexOf"]);
+  }
+
   var length = TO_UINT32(this.length);
   if (length == 0) return -1;
   if (IS_UNDEFINED(index)) {
@@ -1063,6 +1165,11 @@
 
 
 function ArrayLastIndexOf(element, index) {
+  if (IS_NULL_OR_UNDEFINED(this) && !IS_UNDETECTABLE(this)) {
+    throw MakeTypeError("called_on_null_or_undefined",
+                        ["Array.prototype.lastIndexOf"]);
+  }
+
   var length = TO_UINT32(this.length);
   if (length == 0) return -1;
   if (%_ArgumentsLength() < 2) {
@@ -1116,6 +1223,11 @@
 
 
 function ArrayReduce(callback, current) {
+  if (IS_NULL_OR_UNDEFINED(this) && !IS_UNDETECTABLE(this)) {
+    throw MakeTypeError("called_on_null_or_undefined",
+                        ["Array.prototype.reduce"]);
+  }
+
   if (!IS_FUNCTION(callback)) {
     throw MakeTypeError('called_non_callable', [callback]);
   }
@@ -1145,6 +1257,11 @@
 }
 
 function ArrayReduceRight(callback, current) {
+  if (IS_NULL_OR_UNDEFINED(this) && !IS_UNDETECTABLE(this)) {
+    throw MakeTypeError("called_on_null_or_undefined",
+                        ["Array.prototype.reduceRight"]);
+  }
+
   if (!IS_FUNCTION(callback)) {
     throw MakeTypeError('called_non_callable', [callback]);
   }