PLT Freezing on UART command with "extract"

I created a PLT test plan that includes a step like this:

- title: Example
  steps:
  - uartcmd: uart UART0
    send: "a command;\r\n"
    expect: "a response;"

This command works fine. However, when I add an "extract" statement, like so:

- title: Example
  steps:
  - uartcmd: uart UART0
    send: "a command;\r\n"
    expect: "a response;"
    extract: "([^;]*)"
    extractKey: A_RESPONSE

... the PLT freezes on the UART command. The test plan does not time out (I have let it set for about 10 minutes now, and it is still stuck).