diff --git a/Foundation/CPArray/_CPJavaScriptArray.j b/Foundation/CPArray/_CPJavaScriptArray.j
index 41653919b..454d5a670 100644
--- a/Foundation/CPArray/_CPJavaScriptArray.j
+++ b/Foundation/CPArray/_CPJavaScriptArray.j
@@ -360,4 +360,15 @@ var concat = Array.prototype.concat,
 
 @end
 
-Array.prototype.isa = _CPJavaScriptArray;
+if (Array.prototype.isa !== _CPJavaScriptArray)
+{
+    Object.defineProperties(Array.prototype,
+    {
+        isa:
+        {
+            value: _CPJavaScriptArray,
+            enumerable: false,
+            writable: true
+        }
+    });
+}
diff --git a/Foundation/CPData.j b/Foundation/CPData.j
index 1412e034a..78478cff3 100644
--- a/Foundation/CPData.j
+++ b/Foundation/CPData.j
@@ -214,5 +214,27 @@
 
 @end
 
-CFData.prototype.isa = CPData;
-CFMutableData.prototype.isa = CPData;
+if (CFData.prototype.isa !== CPData)
+{
+    Object.defineProperties(CFData.prototype,
+    {
+        isa:
+        {
+            value: CPData,
+            enumerable: false,
+            writable: true
+        }
+    })
+};
+if (CFMutableData.prototype.isa !== CPData)
+{
+    Object.defineProperties(CFMutableData.prototype,
+    {
+        isa:
+        {
+            value: CPData,
+            enumerable: false,
+            writable: true
+        }
+    });
+}
diff --git a/Foundation/CPDate.j b/Foundation/CPDate.j
index 000a2c849..79efc26ca 100644
--- a/Foundation/CPDate.j
+++ b/Foundation/CPDate.j
@@ -286,7 +286,18 @@ Date.parseISO8601 = function (date)
     return timestamp;
 };
 
-Date.prototype.isa = CPDate;
+if (Date.prototype.isa !== CPDate)
+{
+    Object.defineProperties(Date.prototype,
+    {
+        isa:
+        {
+            value: CPDate,
+            enumerable: false,
+            writable: true
+        }
+    });
+}
 
 function _isNumberType(value)
 {
diff --git a/Foundation/CPDictionary.j b/Foundation/CPDictionary.j
index 1238f1e09..d2cd8f229 100755
--- a/Foundation/CPDictionary.j
+++ b/Foundation/CPDictionary.j
@@ -751,5 +751,27 @@ var CPDictionaryMaxDescriptionRecursion = 10;
 
 @end
 
-CFDictionary.prototype.isa = CPDictionary;
-CFMutableDictionary.prototype.isa = CPMutableDictionary;
+if (CFDictionary.prototype.isa !== CPDictionary)
+{
+    Object.defineProperties(CFDictionary.prototype,
+    {
+        isa:
+        {
+            value: CPDictionary,
+            enumerable: false,
+            writable: true
+        }
+    });
+}
+if (CFMutableDictionary.prototype.isa !== CPMutableDictionary)
+{
+    Object.defineProperties(CFMutableDictionary.prototype,
+    {
+        isa:
+        {
+            value: CPMutableDictionary,
+            enumerable: false,
+            writable: true
+        }
+    });
+}
\ No newline at end of file
diff --git a/Foundation/CPError.j b/Foundation/CPError.j
index edc270514..8e120ca9d 100644
--- a/Foundation/CPError.j
+++ b/Foundation/CPError.j
@@ -166,5 +166,16 @@ var CPErrorCodeKey = @"CPErrorCodeKey",
 
 @end
 
-CFError.prototype.isa = CPError;
+if (CFError.prototype.isa !== CPError)
+{
+    Object.defineProperties(CFError.prototype,
+    {
+        isa:
+        {
+            value: CPError,
+            enumerable: false,
+            writable: true
+        }
+    });
+}
 
diff --git a/Foundation/CPException.j b/Foundation/CPException.j
index 70064a984..392cbe1b0 100755
--- a/Foundation/CPException.j
+++ b/Foundation/CPException.j
@@ -222,8 +222,30 @@ var CPExceptionNameKey      = @"CPExceptionNameKey",
 
 // toll-free bridge Error to CPException
 // [CPException alloc] uses an objj_exception, which is a subclass of Error
-Error.prototype.isa = CPException;
-Error.prototype._userInfo = null;
+if (Error.prototype.isa !== CPException)
+{
+    Object.defineProperties(Error.prototype,
+    {
+        isa:
+        {
+            value: CPException,
+            enumerable: false,
+            writable: true
+        }
+    });
+}
+if (Error.prototype._userInfo !== null)
+{
+    Object.defineProperties(Error.prototype,
+    {
+        _userInfo:
+        {
+            value: null,
+            enumerable: false,
+            writable: true
+        }
+    });
+}
 
 [CPException initialize];
 
diff --git a/Foundation/CPNumber.j b/Foundation/CPNumber.j
index 0a45c2bd2..c14caf47c 100644
--- a/Foundation/CPNumber.j
+++ b/Foundation/CPNumber.j
@@ -346,6 +346,29 @@ FIXME: Do we need this?
 
 @end
 
-Number.prototype.isa = CPNumber;
-Boolean.prototype.isa = CPNumber;
+if (Number.prototype.isa !== CPNumber)
+{
+    Object.defineProperties(Number.prototype,
+    {
+        isa:
+        {
+            value: CPNumber,
+            enumerable: false,
+            writable: true
+        }
+    });
+}
+if (Boolean.prototype.isa !== CPNumber)
+{
+    Object.defineProperties(Boolean.prototype,
+    {
+        isa:
+        {
+            value: CPNumber,
+            enumerable: false,
+            writable: true
+        }
+    });
+}
+
 [CPNumber initialize];
diff --git a/Foundation/CPString.j b/Foundation/CPString.j
index eeccf81b5..e642bab49 100644
--- a/Foundation/CPString.j
+++ b/Foundation/CPString.j
@@ -1010,4 +1010,15 @@ String.prototype.stripDiacritics = function()
     return output;
 };
 
-String.prototype.isa = CPString;
+if (String.prototype.isa !== CPString)
+{
+    Object.defineProperties(String.prototype,
+    {
+        isa:
+        {
+            value: CPString,
+            enumerable: false,
+            writable: true
+        }
+    })
+};
diff --git a/Foundation/CPURL.j b/Foundation/CPURL.j
index dd69280c5..8202fdbc8 100644
--- a/Foundation/CPURL.j
+++ b/Foundation/CPURL.j
@@ -260,4 +260,15 @@ var CPURLURLStringKey   = @"CPURLURLStringKey",
 
 @end
 
-CFURL.prototype.isa = [CPURL class];
+if (CFURL.prototype.isa !== CPURL)
+{
+    Object.defineProperties(CFURL.prototype,
+    {
+        isa:
+        {
+            value: CPURL,
+            enumerable: false,
+            writable: true
+        }
+    })
+};
diff --git a/Tests/Foundation/CPArrayTest.j b/Tests/Foundation/CPArrayTest.j
index 828b70efe..850af1cbd 100644
--- a/Tests/Foundation/CPArrayTest.j
+++ b/Tests/Foundation/CPArrayTest.j
@@ -722,6 +722,22 @@
     }];
 }
 
+// As we have a tool-free bridges between many classes and JavaScript types by setting the isa property
+// on the type, for example Array and CPArray. Using the for...in enumeration the isa property should not
+// be included in the loop as the property is now not enumerable.
+- (void)testLoopingArrayWithForIn
+{
+    let array = @[@"a", @"b", @"c", @"d"],
+        i = 0;
+
+    for (const element in array) {
+        i = i + 1;
+        element + element; // Remove warning that the variable is never read
+    }
+
+    [self assert:i equals:4];
+}
+
 @end
 
 @implementation AlwaysEqual : CPObject