diff --git a/core/modules/system/src/Tests/Plugin/Discovery/DiscoveryTestBase.php b/core/modules/system/src/Tests/Plugin/Discovery/DiscoveryTestBase.php
index 31b69b7..6313ce1 100644
--- a/core/modules/system/src/Tests/Plugin/Discovery/DiscoveryTestBase.php
+++ b/core/modules/system/src/Tests/Plugin/Discovery/DiscoveryTestBase.php
@@ -46,7 +46,9 @@ function testDiscoveryInterface() {
     // Ensure that getDefinitions() returns the expected definitions.
     // For the arrays to be identical (instead of only equal), they must be
     // sorted equally, which seems unnecessary here.
-    $this->assertEqual($this->discovery->getDefinitions(), $this->expectedDefinitions);
+    // The discovered definitions may contain circular references; use a custom
+    // assertion message to prevent var_export() from getting called.
+    $this->assertEqual($this->discovery->getDefinitions(), $this->expectedDefinitions, 'Expected definitions found.');
 
     // Ensure that getDefinition() returns the expected definition.
     foreach ($this->expectedDefinitions as $id => $definition) {