This Deployment looks fine, but kubectl apply rejects it. The two env values, FEATURE_ENABLED set to true and PORT set to 8080, trigger an error about failing to unmarshal a bool into a string field. Unmarshal just means converting parsed data into a typed structure. The exact wording varies by version.

Two checks happen, in order. First, the YAML parser, the tool that reads your text, guesses a type for each value. It decides true is a boolean and 8080 is a number. Then the env schema, the API's rulebook for each field, requires a string. The types don't match, so it's rejected.

Look at the two sides. On the left, the text is unquoted, so the parser tags true as a boolean and 8080 as a number, and the schema gate stops them. On the right, quotes tell the parser to keep the text as text. Now both arrive as strings and pass.

Here's the fix on the same two variables. FEATURE_ENABLED and PORT are unchanged, except the values now sit inside double quotes. The parser keeps them as text, so the schema check passes and kubectl apply reports the deployment as configured. Single quotes work too for plain strings like these.

This trap isn't limited to env. ConfigMap data values must be strings, and so must label and annotation values. Also, older YAML 1.1 parsers may read yes, no, on, and off as booleans. When in doubt, quote them.

Helm is a tool that templates Kubernetes manifests. In values.yaml, unquoted 1.10 is parsed as the number 1.1. The quote function then renders "1.1", which is a valid string but the wrong version. Quoting in the template can't undo that. Quote "1.10" at its source in values.yaml.

Follow the path left to right. In values.yaml, 1.10 is parsed as the number 1.1, so the trailing zero is already gone. The template renders 1.1, and that's still a valid manifest. Nothing complains, because a correct-looking value passes validation. It gets applied, and the wrong version is what actually deploys.

On the left, kubectl apply stops with a red error. Nothing deploys, and you fix the quotes. On the right, the manifest applies cleanly, but the app runs version 1.1 instead of 1.10. The loud error is safer, because it blocks the bad change. The silent one passes every check and surfaces later in production.

Quoting only fixes the YAML. It doesn't change what your app receives. The env var reaches the container as text, and the app code reads it as a string. If you need a boolean or a number, your code has to parse "true" or "8080" itself. That step always belongs to the app.

YAML guesses each value's type first; Kubernetes then demands strings. So quote anything that must be a string, at its source. Before you apply, check the rendered output with helm template or a dry run, and catch the surprise early.