diff --git a/Source/SocketIO/Engine/SocketEngine.swift b/Source/SocketIO/Engine/SocketEngine.swift
index 969011b..d6994a2 100644
--- a/Source/SocketIO/Engine/SocketEngine.swift
+++ b/Source/SocketIO/Engine/SocketEngine.swift
@@ -223,7 +223,11 @@ public final class SocketEngine : NSObject, URLSessionDelegate, SocketEnginePoll
 
     private func handleBase64(message: String) {
         // binary in base64 string
-        let noPrefix = String(message[message.index(message.startIndex, offsetBy: 2)..<message.endIndex])!
+        guard let noPrefix = String(message[message.index(message.startIndex, offsetBy: 2)..<message.endIndex]) else {
+            client?.parseEngineBinaryData(Data())
+
+            return
+        }
 
         if let data = Data(base64Encoded: noPrefix, options: .ignoreUnknownCharacters) {
             client?.parseEngineBinaryData(data)