Field name starting with number crashes react-final-form in validate or onChange event

### Are you submitting a **bug report** or a **feature request**?

bug report

### What is the current behavior?

If the field name starts with a number, react-final-form crashes on validate step or on change

### What is the expected behavior?

Field with name "02" or "03" expected to work in the same way as "a02" or "b03"

### Sandbox Link

### What's your environment?

react-final-form 6.4.0, webpack 4.42.1

### Other information