Index: src/wp-content/themes/twentyfifteen/functions.php
===================================================================
--- src/wp-content/themes/twentyfifteen/functions.php	(revision 30827)
+++ src/wp-content/themes/twentyfifteen/functions.php	(working copy)
@@ -181,10 +181,16 @@
 		$subsets .= ',vietnamese';
 	}
 
-	return add_query_arg( array(
-		'family' => urlencode( implode( '|', $fonts ) ),
-		'subset' => urlencode( $subsets ),
-	), '//fonts.googleapis.com/css' );
+	$font_url = '';
+
+	if ( $fonts ) {
+		$font_url = add_query_arg( array(
+			'family' => urlencode( implode( '|', $fonts ) ),
+			'subset' => urlencode( $subsets ),
+		), '//fonts.googleapis.com/css' );
+	}
+
+	return $font_url;
 }
 endif;