diff --git a/tools/ui/embed.cpp b/tools/ui/embed.cpp
--- a/tools/ui/embed.cpp
+++ b/tools/ui/embed.cpp
@@ -147,9 +147,9 @@ int main(int argc, char ** argv) {
 
     const std::string out_cpp   = argv[1];
     const std::string out_h     = argv[2];
-    const std::string asset_dir = argv[3];
+    const std::string asset_dir = argc == 4 ? argv[3] : "";
 
-    const bool        use_gzip = std::filesystem::exists(asset_dir + "/_gzip");
+    const bool        use_gzip = !asset_dir.empty() && std::filesystem::exists(asset_dir + "/_gzip");
     const std::string in_dir   = use_gzip ? (asset_dir + "/_gzip") : asset_dir;
 
     std::vector<asset_entry> assets;