diff --git a/src/main/java/org/embeddedt/modernfix/common/mixin/perf/fix_handshake_stall/HandshakeHandlerMixin.java b/src/main/java/org/embeddedt/modernfix/common/mixin/perf/fix_handshake_stall/HandshakeHandlerMixin.java
index a3c8f503..ef2b13be 100644
--- a/src/main/java/org/embeddedt/modernfix/common/mixin/perf/fix_handshake_stall/HandshakeHandlerMixin.java
+++ b/src/main/java/org/embeddedt/modernfix/common/mixin/perf/fix_handshake_stall/HandshakeHandlerMixin.java
@@ -8,8 +8,11 @@ import net.minecraftforge.network.NetworkRegistry;
 import org.spongepowered.asm.mixin.Mixin;
 import org.spongepowered.asm.mixin.Shadow;
 import org.spongepowered.asm.mixin.injection.At;
+import org.spongepowered.asm.mixin.injection.Inject;
 import org.spongepowered.asm.mixin.injection.Slice;
+import org.spongepowered.asm.mixin.injection.callback.CallbackInfo;
 
+import java.util.Collections;
 import java.util.List;
 
 @Mixin(value = HandshakeHandler.class, remap = false)
@@ -23,6 +26,16 @@ public class HandshakeHandlerMixin {
     @Shadow
     private List<Integer> sentMessages;
 
+    /**
+     * @author embeddedt
+     * @reason we must synchronize sentMessages because it is modified from both the Netty thread and the
+     * server thread
+     */
+    @Inject(method = "<init>", at = @At("RETURN"))
+    private void synchronizeSentMessages(CallbackInfo ci) {
+        this.sentMessages = Collections.synchronizedList(this.sentMessages);
+    }
+
     /**
      * @author embeddedt
      * @reason Forge only sends one login payload per tick. It takes many seconds to send all the payloads at this rate.