Firebase Storage's OnSuccessListener callback not calling

When uploading files through Firebase Storage, the onSuccess method isn't calling.

I'm currently running Android Studio 3.0 Canary 2, with 'com.google.firebase:firebase-storage:10.2.6'.

fun uploadImage(pathToImage: String, downloadCallback: FirebaseCallback<String?>) {
        val file = Uri.fromFile(File(pathToImage))
        val ref = mStorRef.child("images/"+file.lastPathSegment)

        ref.putFile(file).addOnSuccessListener {
            object : OnSuccessListener<UploadTask.TaskSnapshot> {
                override fun onSuccess(taskSnapshot: UploadTask.TaskSnapshot?) {
                    val url = taskSnapshot?.downloadUrl
                    Log.d("FirebaseManager", "Upload Successful")
                    downloadCallback.callback(url.toString())
                }
            }
        }
    }