diff --git a/tests/before_script.sh b/tests/before_script.sh
index e0ea326..d18187e 100755
--- a/tests/before_script.sh
+++ b/tests/before_script.sh
@@ -91,6 +91,14 @@ EOF
     sleep 5
     sudo service nginx start
     phpunit_shim
+
+    # Use an older version of PHPUnit for HHVM builds so that unit tests can be
+    # started. HHVM 3.18 (PHP_VERSION=PHP 5.6.99-hhvm) is the last version to
+    # run on Trusty yet PHPUnit 6 requires PHP 7.0 or PHP 7.1.
+    # Avoids error:
+    #   This version of PHPUnit is supported on PHP 7.0 and PHP 7.1.
+    #   You are using PHP 5.6.99-hhvm (/usr/bin/hhvm).
+    composer require phpunit/phpunit:5.7
 elif [[ "${TRAVIS_PHP_VERSION}" == "nightly" ]]; then
     php -S 127.0.0.1:8000 -t tests/PHPCurlClass/ &
 fi