diff --git a/tiny_gltf.h b/tiny_gltf.h
index 6fc7b8a..40f1d38 100644
--- a/tiny_gltf.h
+++ b/tiny_gltf.h
@@ -793,7 +793,7 @@ struct FsCallbacks
     ReadWholeFileFunction ReadWholeFile;
     WriteWholeFileFunction WriteWholeFile;
 
-    void* user_data = nullptr; // An argument that is passed to all fs callbacks
+    void* user_data; // An argument that is passed to all fs callbacks
 };
 
 #ifndef TINYGLTF_NO_FS