diff --git a/src/wp-includes/functions.php b/src/wp-includes/functions.php
index fbe1219866..42f6873b64 100644
--- a/src/wp-includes/functions.php
+++ b/src/wp-includes/functions.php
@@ -782,7 +782,7 @@ function add_query_arg() {
 	$qs = urlencode_deep( $qs ); // this re-URL-encodes things that were already in the query string
 	if ( is_array( $args[0] ) ) {
 		$kayvees = $args[0];
-		$qs = array_merge( $qs, $kayvees );
+		$qs = array_replace( $qs, $kayvees );
 	} else {
 		$qs[ $args[0] ] = $args[1];
 	}
diff --git a/tests/phpunit/tests/functions.php b/tests/phpunit/tests/functions.php
index 527780ae27..4d3a4bbdd5 100644
--- a/tests/phpunit/tests/functions.php
+++ b/tests/phpunit/tests/functions.php
@@ -258,6 +258,20 @@ class Tests_Functions extends WP_UnitTestCase {
 		$_SERVER['REQUEST_URI'] = $old_req_uri;
 	}
 
+	/**
+	 * @ticket 31306
+	 */
+	function test_add_query_arg_numeric_keys() {
+		$url = add_query_arg( array( 'foo' => 'bar' ), '1=1' );
+		$this->assertEquals('1=1&foo=bar', $url);
+
+		$url = add_query_arg( array( 'foo' => 'bar', '1' => '2' ), '1=1' );
+		$this->assertEquals('1=2&foo=bar', $url);
+
+		$url = add_query_arg( array( '1' => '2' ), 'foo=bar' );
+		$this->assertEquals('foo=bar&1=2', $url);
+	}
+
 	/**
 	 * @ticket 21594
 	 */