--- a/toolkit/components/telemetry/docs/core-ping.rst
+++ b/toolkit/components/telemetry/docs/core-ping.rst
@@ -73,16 +73,19 @@ engine from the search plugins (in order
 
 * In the distribution
 * From the localized plugins shipped with the browser
 * The third-party plugins that are installed in the profile directory
 
 If the plugins fail to create a search engine instance, this field is also
 ``null``.
 
+This field can also be ``null`` when a custom search engine is set as the
+default.
+
 profileDate
 ~~~~~~~~~~~
 On Android, this value is created at profile creation time and retrieved or,
 for legacy profiles, taken from the package install time (note: this is not the
 same exact metric as profile creation time but we compromised in favor of ease
 of implementation).
 
 Additionally on Android, this field may be ``null`` in the unlikely event that