Check for string-type parameter VALUE

[phil-davis]

Rebase of #636

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.78%. Comparing base (a2f05dd) to head (68d7d9b).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #640   +/-   ##
=========================================
  Coverage     98.78%   98.78%           
- Complexity     1875     1877    +2     
=========================================
  Files            71       71           
  Lines          5270     5273    +3     
=========================================
+ Hits           5206     5209    +3     
  Misses           64       64           

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[phil-davis]

CI passes, so existing test cases are still OK.

Need a response to #636 (comment) to be able to add a useful extra unit test case.