diff --git a/src/applications/harbormaster/step/HarbormasterBuildStepImplementation.php b/src/applications/harbormaster/step/HarbormasterBuildStepImplementation.php
--- a/src/applications/harbormaster/step/HarbormasterBuildStepImplementation.php
+++ b/src/applications/harbormaster/step/HarbormasterBuildStepImplementation.php
@@ -193,7 +193,7 @@
    * @return string String with variables replaced safely into it.
    */
   protected function mergeVariables($function, $pattern, array $variables) {
-    $regexp = '@\\$\\{(?P<name>[a-z\\./-]+)\\}@';
+    $regexp = '@\\$\\{(?P<name>[a-z\\./_-]+)\\}@';
 
     $matches = null;
     preg_match_all($regexp, $pattern, $matches);