diff --git a/src/main/java/dan200/computercraft/shared/util/IDAssigner.java b/src/main/java/dan200/computercraft/shared/util/IDAssigner.java
index fd0f222ab..fbbf461bf 100644
--- a/src/main/java/dan200/computercraft/shared/util/IDAssigner.java
+++ b/src/main/java/dan200/computercraft/shared/util/IDAssigner.java
@@ -36,7 +36,7 @@ public final class IDAssigner
         return getNextID( file, false );
     }
 
-    private static int getNextID( File location, boolean directory )
+    private static synchronized int getNextID( File location, boolean directory )
     {
         // Determine where to locate ID file
         File lastIdFile;