v4 CLI: --watch --map appends "undefined" to output; --map <file> triggers TypeError [ERR_INVALID_ARG_TYPE]

[RV7PR]

What version of Tailwind CSS are you using?
v4.1.17

What build tool (or framework if it abstracts the build tool) are you using?
Tailwind CLI (via npx @tailwindcss/cli or bunx)

What version of Node.js are you using?
Node.js v25.2.1 Bun v1.3.3

What browser are you using?
N/A (CLI issue)

What operating system are you using?
macOS (MacBook Pro 16-inch 2023)

Minimal repro:

Empty input.css and add simple class while watching.

Bun:
bunx @tailwindcss/cli -i ./src/input.css -o ./dist/output.css --watch --map ./dist/output.css.map
bunx @tailwindcss/cli -i ./src/input.css -o ./dist/output.css --watch --map

Node:
bunx @tailwindcss/cli -i ./src/input.css -o ./dist/output.css --watch --map ./dist/output.css.map
bunx @tailwindcss/cli -i ./src/input.css -o ./dist/output.css --watch --map

Describe your issue
In watch mode with --map:

• Inline (--map): Corrupts output.css end with "undefined" on changes.
• Separate file (--map ./output.css.map): Crashes with TypeError [ERR_INVALID_ARG_TYPE]: The "data" argument must be of type string or an instance of Buffer, TypedArray, or DataView. Received undefined.