diff --git a/omaha_request_params.h b/omaha_request_params.h
index 9e41716..6284ee1 100644
--- a/omaha_request_params.h
+++ b/omaha_request_params.h
@@ -259,8 +259,13 @@
   FRIEND_TEST(OmahaRequestParamsTest, SetTargetChannelInvalidTest);
   FRIEND_TEST(OmahaRequestParamsTest, ToMoreStableChannelFlagTest);
 
-  // Returns true if |channel| is a valid channel, false otherwise.
-  bool IsValidChannel(const std::string& channel) const;
+  // Returns true if |channel| is a valid channel, otherwise write error to
+  // |error_message| if passed and return false.
+  bool IsValidChannel(const std::string& channel,
+                      std::string* error_message) const;
+  bool IsValidChannel(const std::string& channel) const {
+    return IsValidChannel(channel, nullptr);
+  }
 
   // Returns the index of the given channel.
   int GetChannelIndex(const std::string& channel) const;