diff --git a/npm_install.py b/npm_install.py
index f578763..309206e 100644
--- a/npm_install.py
+++ b/npm_install.py
@@ -588,13 +588,24 @@ def _build_frontend(src_frontend: Path, dest_frontend: Path):
         import shutil as _s
         return _s.which(cmd) is not None
 
-    yarn = "yarn" if _which("yarn") else ("yarnpkg" if _which("yarnpkg") else "")
+    def _good_yarn(cmd: str) -> bool:
+        out = (run_out([cmd, "--version"], check=False) or "").strip()
+        return bool(re.match(r"^\d+(?:\.\d+){1,3}$", out))
+
+    yarn = None
+    for cand in ("yarn", "yarnpkg"):
+        if _which(cand) and _good_yarn(cand):
+            yarn = cand
+            break
     if not yarn:
         run(["corepack", "enable"], check=False)
         run(["corepack", "prepare", "yarn@stable", "--activate"], check=False)
-        yarn = "yarn" if _which("yarn") else ("yarnpkg" if _which("yarnpkg") else "")
+        for cand in ("yarn", "yarnpkg"):
+            if _which(cand) and _good_yarn(cand):
+                yarn = cand
+                break
     if not yarn:
-        raise RuntimeError("Brak Yarn. Zainstaluj: npm i -g yarn  (lub użyj corepack).")
+        raise RuntimeError("No yarn in system")
 
     with step("Installing frontend dependencies (yarn)"):
         os.environ["NODE_ENV"] = "development"
@@ -602,8 +613,8 @@ def _build_frontend(src_frontend: Path, dest_frontend: Path):
         cache_dir = (run_out([yarn, "cache", "dir"], check=False) or "").strip()
         if cache_dir and not Path(cache_dir).exists():
             Path(cache_dir).mkdir(parents=True, exist_ok=True)
-        run([yarn, "cache", "clean"], check=False)
 
+        run([yarn, "cache", "clean"], check=False)
         run([yarn, "install"])
 
     with step("Building frontend (yarn build)"):