diff --git a/internal/reader/rss/adapter.go b/internal/reader/rss/adapter.go
index 36b39da4..273209ef 100644
--- a/internal/reader/rss/adapter.go
+++ b/internal/reader/rss/adapter.go
@@ -35,8 +35,8 @@ func (r *RSSAdapter) BuildFeed(baseURL string) *model.Feed {
 	}
 
 	// Ensure the Site URL is absolute.
-	if siteURL, err := urllib.AbsoluteURL(baseURL, feed.SiteURL); err == nil {
-		feed.SiteURL = siteURL
+	if absoluteSiteURL, err := urllib.AbsoluteURL(baseURL, feed.SiteURL); err == nil {
+		feed.SiteURL = absoluteSiteURL
 	}
 
 	// Try to find the feed URL from the Atom links.
diff --git a/internal/urllib/url.go b/internal/urllib/url.go
index 55230187..56d41d96 100644
--- a/internal/urllib/url.go
+++ b/internal/urllib/url.go
@@ -21,7 +21,10 @@ func IsAbsoluteURL(link string) bool {
 // AbsoluteURL converts the input URL as absolute URL if necessary.
 func AbsoluteURL(baseURL, input string) (string, error) {
 	if strings.HasPrefix(input, "//") {
-		input = "https://" + input[2:]
+		return "https:" + input, nil
+	}
+	if strings.HasPrefix(input, "https://") || strings.HasPrefix(input, "http://") {
+		return input, nil
 	}
 
 	u, err := url.Parse(input)