diff --git a/source/scriptinterface/ScriptInterface.cpp b/source/scriptinterface/ScriptInterface.cpp
index babca761df..53621fe7b3 100644
--- a/source/scriptinterface/ScriptInterface.cpp
+++ b/source/scriptinterface/ScriptInterface.cpp
@@ -792,6 +792,8 @@ bool ScriptInterface::LoadScript(const VfsPath& filename, const std::wstring& co
 
 bool ScriptInterface::LoadGlobalScriptFile(const VfsPath& path)
 {
+	debug_assert(ThreadUtil::IsMainThread()); // VFS isn't thread-safe
+
 	if (!VfsFileExists(g_VFS, path))
 	{
 		LOGERROR(L"File '%ls' does not exist", path.string().c_str());
@@ -878,6 +880,8 @@ CScriptValRooted ScriptInterface::ParseJSON(const std::string& string_utf8)
 
 CScriptValRooted ScriptInterface::ReadJSONFile(const VfsPath& path)
 {
+	debug_assert(ThreadUtil::IsMainThread()); // VFS isn't thread-safe
+
 	if (!VfsFileExists(g_VFS, path))
 	{
 		LOGERROR(L"File '%ls' does not exist", path.string().c_str());