diff --git a/README.md b/README.md
index 17a3b754..f5e688ce 100644
--- a/README.md
+++ b/README.md
@@ -23,6 +23,10 @@ multisite, this will just be the tables for the current site unless
 Search/replace intelligently handles PHP serialized data, and does not
 change primary key values.
 
+Tables without a primary key are skipped. To check whether a table has a
+primary key, run `wp db query 'DESCRIBE <table>'` and look for 'PRI' in
+the Key column.
+
 **OPTIONS**
 
 	[<old>]
diff --git a/src/Search_Replace_Command.php b/src/Search_Replace_Command.php
index 04a55dfc..c37ad0df 100644
--- a/src/Search_Replace_Command.php
+++ b/src/Search_Replace_Command.php
@@ -140,6 +140,10 @@ class Search_Replace_Command extends WP_CLI_Command {
 	 * Search/replace intelligently handles PHP serialized data, and does not
 	 * change primary key values.
 	 *
+	 * Tables without a primary key are skipped. To check whether a table has a
+	 * primary key, run `wp db query 'DESCRIBE <table>'` and look for 'PRI' in
+	 * the Key column.
+	 *
 	 * ## OPTIONS
 	 *
 	 * [<old>]