"-1" Priority in Agile board's card colours Priority option

Summary

Creating issue with Priority field hidden in either Field Configuration or the Priority field is removed from issue type's screen will cause "-1" Priority to appear in Agile board's card colour priority option.

Steps to reproduce:

1. Select JIRA (Software) Project.
2. Access (Board -> Configure -> Card color) and choose card color option "Priority".
3. Access the Field Configuration used by the newly created project and hide the Priority field
4. Create an issue in the JIRA project
5. Access the newly created the board where the issue is on. Open (Board -> Configure -> Card color) and the "-1" priority -1 will be shown

Environment

JIRA 7.3.0, JIRA 7.3.8

Expected Results

When issues are created with no Priority field entry, "-1" priority is not added to the Card color page.

Actual Results

When issues are created with no Priority field entry, "-1" priority is added to the Card color page.

Workaround

User can potentially add a default priority from workflow post-function