./Build testcover fails

I'm using Perl 5.26 on Mac OS High Sierra.

I created a module using Perl's module-starter command using Build.PL. I created a test file inside of the /t directory which uses Test::More. The tests all pass using the command ./Build test. The coverage test works using Devel::Cover when run using the command cover -test. But when I try to use ./Build testcover, as documented in the Devel::Cover module, it fails with numerous errors of the following form:

Devel::Cover: Warning: can't open Specio::Coercion->_coercion for MD5 digest: No such file or directory
Devel::Cover: Can't find file "Specio::Coercion->_coercion" (Specio::Coercion->_coercion): ignored.

Since Devel::Cover works when run separately, there must be some setting that is missing when run as part of the Build test script but I cannot find anything that works. I cannot find anything online that is similar to this error.

Any help is appreciated.