diff --git a/main.go b/main.go
index 990bd2f..2ff76cc 100644
--- a/main.go
+++ b/main.go
@@ -11,6 +11,7 @@ import (
 	"fmt"
 	"io"
 	"io/fs"
+	"net"
 	"net/http"
 	"net/url"
 	"os"
@@ -464,11 +465,16 @@ func main() {
 	}
 
 	if *serverFlag != "" {
-		var idnaProfile = idna.New(idna.MapForLookup(), idna.BidiRule())
-		// See an explanation in git-pages auth.go for why this replacement is done.
-		if _, err := idnaProfile.ToASCII(strings.ReplaceAll(*serverFlag, "_", "a")); err != nil {
-			fmt.Fprintf(os.Stderr, "error: --server must be used with a domain name: %q\n", *serverFlag)
+		if serverHost, _, err := net.SplitHostPort(*serverFlag); err != nil {
+			fmt.Fprintf(os.Stderr, "error: invalid --server value %q\n", *serverFlag)
 			os.Exit(usageExitCode)
+		} else {
+			var idnaProfile = idna.New(idna.MapForLookup(), idna.BidiRule())
+			// See an explanation in git-pages auth.go for why this replacement is done.
+			if _, err := idnaProfile.ToASCII(strings.ReplaceAll(serverHost, "_", "a")); err != nil {
+				fmt.Fprintf(os.Stderr, "error: --server must be used with a domain name: %q\n", serverHost)
+				os.Exit(usageExitCode)
+			}
 		}
 	}