diff --git a/src/wp-includes/general-template.php b/src/wp-includes/general-template.php
index e8e936cbc4..8a3a71cd6c 100644
--- a/src/wp-includes/general-template.php
+++ b/src/wp-includes/general-template.php
@@ -878,6 +878,10 @@ function get_custom_logo( $blog_id = 0 ) {
 	$custom_logo_id = get_theme_mod( 'custom_logo' );
 	$size           = get_theme_support( 'custom-logo', 'size' );
 
+	if ( ! $size ) {
+		$size = 'full';
+	}
+
 	// We have a logo. Logo is go.
 	if ( $custom_logo_id ) {
 		$html = sprintf( '<a href="%1$s" class="custom-logo-link" rel="home" itemprop="url">%2$s</a>',
diff --git a/tests/phpunit/tests/general/template.php b/tests/phpunit/tests/general/template.php
index c7807e2c28..d50ee884e4 100644
--- a/tests/phpunit/tests/general/template.php
+++ b/tests/phpunit/tests/general/template.php
@@ -280,6 +280,7 @@ class Tests_General_Template extends WP_UnitTestCase {
 		$this->_set_custom_logo();
 		$home_url = get_home_url( $blog_id, '/' );
 		$size     = get_theme_support( 'custom-logo', 'size' );
+		$size     = $size ? $size : 'full';
 		$image    = wp_get_attachment_image( $this->custom_logo_id, $size, false, array(
 			'class'     => "custom-logo attachment-$size",
 			'data-size' => $size,
@@ -302,6 +303,7 @@ class Tests_General_Template extends WP_UnitTestCase {
 
 		$this->_set_custom_logo();
 		$size  = get_theme_support( 'custom-logo', 'size' );
+		$size  = $size ? $size : 'full';
 		$image = wp_get_attachment_image( $this->custom_logo_id, $size, false, array(
 			'class'     => "custom-logo attachment-$size",
 			'data-size' => $size,