diff --git a/ConcurrencyTests/PromiseTests.swift b/ConcurrencyTests/PromiseTests.swift
index aa97ec6..07a8ad0 100644
--- a/ConcurrencyTests/PromiseTests.swift
+++ b/ConcurrencyTests/PromiseTests.swift
@@ -187,7 +187,7 @@ class PromiseTests: QuickSpec {
                     expect(mappedFuture.isComplete).toEventually(beTrue())
                     expect(mappedFuture.succeeded).toEventually(beFalse())
                     expect(mappedFuture.failed).toEventually(beTrue())
-                    expect(item(mappedFuture.error, isA: NSError.self, and: {$0.domain == "Could not map value (3) to type String."})).toEventually(beTrue())
+                    expect(item(mappedFuture.error, isA: NSError.self, and:{ $0.description == "Concurrency: Could not map value (3) to type String." })).toEventually(beTrue())
                     expect(mappedFuture.value).toEventually(beNil())
                 }
             }
diff --git a/Source/ConcurrencyHelpers.swift b/Source/ConcurrencyHelpers.swift
index 858f2ca..08c0c71 100644
--- a/Source/ConcurrencyHelpers.swift
+++ b/Source/ConcurrencyHelpers.swift
@@ -8,8 +8,38 @@ import Foundation
 
 extension NSError {
     static func cantMap<T,Q>(value:T, toType: Q.Type) -> NSError {
+        return CantMapError(value: value, toType: toType)
+    }
+}
+
+fileprivate class CantMapError<Q,T>: NSError {
+    
+    private var descriptionString: String
+    
+    init(value: T, toType: Q.Type) {
         let typeString = String(describing: type(of: Q.self)).replacingOccurrences(of: ".Type", with: "")
-        let description = "Could not map value (\(value)) to type \(typeString)."
-        return NSError(domain: description, code: 0, userInfo: nil)
+        let description = "Concurrency: Could not map value (\(value)) to type \(typeString)."
+        self.descriptionString = description
+        
+        super.init(domain: "com.concurrency.map", code: 0, userInfo: ["description" : description])
+    }
+    
+    required init?(coder aDecoder: NSCoder) {
+        self.descriptionString = "Mapping Error"
+        
+        super.init(coder: aDecoder)
+        
+        if let unarchivedDescription = self.userInfo["description"] as? String {
+            self.descriptionString = unarchivedDescription
+        }
+    }
+    
+    override var description: String {
+        return descriptionString
+    }
+    
+    public static func ==(lhs: CantMapError, rhs: CantMapError) -> Bool {
+        return lhs.description == rhs.description
     }
+    
 }