diff --git a/src/Database/Expression/UnaryExpression.php b/src/Database/Expression/UnaryExpression.php
index <HASH>..<HASH> 100644
--- a/src/Database/Expression/UnaryExpression.php
+++ b/src/Database/Expression/UnaryExpression.php
@@ -17,6 +17,9 @@ namespace Cake\Database\Expression;
 use Cake\Database\ExpressionInterface;
 use Cake\Database\ValueBinder;
 
+/**
+ * An expression object that represents an expression with only a single operand.
+ */
 class UnaryExpression extends QueryExpression {
 
 /**
Improve docs for UnaryExpression.