diff --git a/tests/phpunit/includes/testcase-ajax.php b/tests/phpunit/includes/testcase-ajax.php
index 787e6bf5fd..19a87b2a74 100644
--- a/tests/phpunit/includes/testcase-ajax.php
+++ b/tests/phpunit/includes/testcase-ajax.php
@@ -200,4 +200,25 @@ abstract class WP_Ajax_UnitTestCase extends WP_UnitTestCase {
 		if ( !empty( $buffer ) )
 			$this->_last_response = $buffer;
 	}
+
+	/**
+	 * PHPUnit 6+ compatibility shim.
+	 *
+	 * @param mixed      $exception
+	 * @param string     $message
+	 * @param int|string $code
+	 */
+	public function setExpectedException( $exception, $message = '', $code = null ) {
+		if ( is_callable( 'parent::setExpectedException' ) ) {
+			parent::setExpectedException( $exception, $message, $code );
+		} else {
+			$this->expectException( $exception );
+			if ( '' !== $message ) {
+				$this->expectExceptionMessage( $message );
+			}
+			if ( null !== $code ) {
+				$this->expectExceptionCode( $code );
+			}
+		}
+	}
 }
diff --git a/tests/phpunit/includes/testcase.php b/tests/phpunit/includes/testcase.php
index b559cf6f19..554de7e578 100644
--- a/tests/phpunit/includes/testcase.php
+++ b/tests/phpunit/includes/testcase.php
@@ -415,27 +415,6 @@ class WP_UnitTestCase extends PHPUnit_Framework_TestCase {
 		array_push( $this->expected_doing_it_wrong, $doing_it_wrong );
 	}
 
-	/**
-	 * PHPUnit 6+ compatibility shim.
-	 *
-	 * @param mixed      $exception
-	 * @param string     $message
-	 * @param int|string $code
-	 */
-	public function setExpectedException( $exception, $message = '', $code = null ) {
-		if ( is_callable( 'parent::setExpectedException' ) ) {
-			parent::setExpectedException( $exception, $message, $code );
-		} else {
-			$this->expectException( $exception );
-			if ( '' !== $message ) {
-				$this->expectExceptionMessage( $message );
-			}
-			if ( null !== $code ) {
-				$this->expectExceptionCode( $code );
-			}
-		}
-	}
-
 	function deprecated_function_run( $function ) {
 		if ( ! in_array( $function, $this->caught_deprecated ) )
 			$this->caught_deprecated[] = $function;