diff --git a/Zip/Zip.swift b/Zip/Zip.swift
index 43d804a2aa7fba35f58d74466fa8c873b993c47e..e062b14267567dd51c8f8d1e08c1ca3ac4473627 100644
--- a/Zip/Zip.swift
+++ b/Zip/Zip.swift
@@ -66,7 +66,7 @@ public class Zip {
         guard let path = zipFilePath.path where destination.path != nil else {
             throw ZipError.FileNotFound
         }
-        if fileManager.fileExistsAtPath(path) == false || zipFilePath.pathExtension != "zip" {
+        if fileManager.fileExistsAtPath(path) == false || fileExtensionIsInvalid(zipFilePath.pathExtension) {
             throw ZipError.FileNotFound
         }
         
@@ -303,6 +303,18 @@ public class Zip {
         progressTracker.completedUnitCount = Int64(totalSize)
     }
     
-    
+    /**
+     Check if file extension is invalid.
+     
+     - parameter fileExtension: A file extension.
+     
+     - returns: false if the extension is "zip" or "cbz", otherwise true.
+     */
+    internal class func fileExtensionIsInvalid(fileExtension: String?) -> Bool {
+        
+        guard let fileExtension = fileExtension else { return true }
+        
+        return !["zip", "cbz"].contains(fileExtension)
+    }
     
 }
\ No newline at end of file
diff --git a/ZipTests/ZipTests.swift b/ZipTests/ZipTests.swift
index f80b6e4d47577d7546a43d6523930524a0b9ab85..2b59732e041af05d219f3cdb439b93ba2bdb0cc6 100644
--- a/ZipTests/ZipTests.swift
+++ b/ZipTests/ZipTests.swift
@@ -208,6 +208,16 @@ class ZipTests: XCTestCase {
             XCTFail()
         }
     }
+
+    func testFileExtensionIsInvalidForValidUrl() {
+        let fileUrl = NSURL(string: "file.cbz")
+        let result = Zip.fileExtensionIsInvalid(fileUrl?.pathExtension)
+        XCTAssertFalse(result)
+    }
     
-    
+    func testFileExtensionIsInvalidForInvalidUrl() {
+        let fileUrl = NSURL(string: "file.xyz")
+        let result = Zip.fileExtensionIsInvalid(fileUrl?.pathExtension)
+        XCTAssertTrue(result)
+    }
 }