diff --git a/platformio/app.py b/platformio/app.py
index 7d2f1ea9b7..24feb52363 100644
--- a/platformio/app.py
+++ b/platformio/app.py
@@ -29,7 +29,8 @@
 
 
 def projects_dir_validate(projects_dir):
-    assert os.path.isdir(projects_dir)
+    if not os.path.isdir(projects_dir):
+        raise ValueError("Not a directory: %s" % projects_dir)
     return os.path.abspath(projects_dir)
 
 
diff --git a/tests/misc/test_app.py b/tests/misc/test_app.py
new file mode 100644
index 0000000000..65a312f8c5
--- /dev/null
+++ b/tests/misc/test_app.py
@@ -0,0 +1,48 @@
+# Copyright (c) 2014-present PlatformIO <contact@platformio.org>
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Tests for platformio.app validators."""
+
+import os
+
+import pytest
+
+from platformio import app
+
+
+def test_projects_dir_validate_accepts_existing_directory(tmp_path):
+    """A path that points to an existing directory passes through validation."""
+    result = app.projects_dir_validate(str(tmp_path))
+    assert result == os.path.abspath(str(tmp_path))
+
+
+def test_projects_dir_validate_raises_on_nonexistent_path(tmp_path):
+    """
+    Regression for platformio-core#5403: validation used \`assert\` which is
+    stripped by python -O, so invalid paths were silently accepted.
+
+    Use a deliberately bogus subpath under tmp_path so the test is hermetic.
+    """
+    missing = tmp_path / "definitely-does-not-exist"
+    assert not missing.exists()
+    with pytest.raises(ValueError):
+        app.projects_dir_validate(str(missing))
+
+
+def test_projects_dir_validate_raises_on_file_path(tmp_path):
+    """Files are not directories and must be rejected."""
+    a_file = tmp_path / "not_a_dir.txt"
+    a_file.write_text("x")
+    with pytest.raises(ValueError):
+        app.projects_dir_validate(str(a_file))