Custom field not editable in test execution

[ibreestraat]

I created a custom field that I want to enter while execution the test cases.

Custom field:
Name: Measurement 1
Label: Measurement 1
Available for:Test case
Type: Numeric
Enable on: Test execution

This custom field is visible when execution the test and I can step into the field, but it is not possible to enter any number or text.
Is my configuration wrong?