diff --git a/src/TestSessionEnvironment.php b/src/TestSessionEnvironment.php
index c4e60ca..19f945c 100644
--- a/src/TestSessionEnvironment.php
+++ b/src/TestSessionEnvironment.php
@@ -249,7 +249,7 @@ class TestSessionEnvironment
                 $state->database = $dbName; // In case it's changed by the call to SapphireTest::create_temp_db();
 
                 // Set existing one, assumes it already has been created
-                $prefix = defined('SS_DATABASE_PREFIX') ? SS_DATABASE_PREFIX : 'ss_';
+                $prefix = getenv('SS_DATABASE_PREFIX') ?: 'ss_';
                 $pattern = strtolower(sprintf('#^%stmpdb.*#', preg_quote($prefix, '#')));
                 if (!preg_match($pattern, $dbName)) {
                     throw new InvalidArgumentException("Invalid database name format");