diff --git a/plugins/optimization-detective/tests/test-optimization.php b/plugins/optimization-detective/tests/test-optimization.php
index a1a621fac5..f212be8415 100644
--- a/plugins/optimization-detective/tests/test-optimization.php
+++ b/plugins/optimization-detective/tests/test-optimization.php
@@ -109,6 +109,8 @@ function ( $buffer ) use ( $template_override, $filter_override, &$filter_count
 		ob_clean(); // Note the lack of flush here.
 		echo $template_override; // This should get passed into the od_template_output_buffer filter.
 
+		ob_end_flush();
+
 		$buffer = ob_get_clean(); // Get the buffer from our wrapper output buffer.
 
 		$this->assertSame( 1, $filter_count, 'Expected filter to be called once.' );