Incompatible change in behavior of REST API Sprint field

REST API is update issue: PUT /rest/api/latest/issue
Before 7.1 the Sprint custom field (com.pyxis.greenhopper.jira:gh-sprint) requires a string value representing the id of the sprint.
On 7.1, setting the value to a number or string id gives a internal server error with java.lang.ClassCastException: java.util.ArrayList cannot be cast to java.lang.String
On 7.2 a numeric is expected: Number value expected as the Sprint id

REST API should be upward compatible and support both numeric and string forms.

In both cases the field has metadata with string item type

Name  . . . . . . . . . . . . : Sprint
Description . . . . . . . . . : 
Required  . . . . . . . . . . : false
Has default . . . . . . . . . : false
Has allowed values  . . . . . : false
Type  . . . . . . . . . . . . : array
Item type . . . . . . . . . . : string
Custom  . . . . . . . . . . . : true
Custom type . . . . . . . . . : com.pyxis.greenhopper.jira:gh-sprint