diff --git a/gpcm/error.go b/gpcm/error.go
index 84dd516..71b56c0 100644
--- a/gpcm/error.go
+++ b/gpcm/error.go
@@ -7,16 +7,16 @@ import (
 )
 
 type GPError struct {
-	errorCode   int
-	errorString string
-	fatal       bool
+	ErrorCode   int
+	ErrorString string
+	Fatal       bool
 }
 
 func MakeGPError(errorCode int, errorString string, fatal bool) GPError {
 	return GPError{
-		errorCode:   errorCode,
-		errorString: errorString,
-		fatal:       fatal,
+		ErrorCode:   errorCode,
+		ErrorString: errorString,
+		Fatal:       fatal,
 	}
 }
 
@@ -144,12 +144,12 @@ func (err GPError) GetMessage() string {
 		Command:      "error",
 		CommandValue: "",
 		OtherValues: map[string]string{
-			"err":    strconv.Itoa(err.errorCode),
-			"errmsg": err.errorString,
+			"err":    strconv.Itoa(err.ErrorCode),
+			"errmsg": err.ErrorString,
 		},
 	}
 
-	if err.fatal {
+	if err.Fatal {
 		command.OtherValues["fatal"] = ""
 	}
 
@@ -157,6 +157,6 @@ func (err GPError) GetMessage() string {
 }
 
 func (g *GameSpySession) replyError(err GPError) {
-	logging.Error(g.ModuleName, "Reply error:", err.errorString)
+	logging.Error(g.ModuleName, "Reply error:", err.ErrorString)
 	g.Conn.Write([]byte(err.GetMessage()))
 }
diff --git a/gpcm/login.go b/gpcm/login.go
index 38f0ab0..a0b7067 100644
--- a/gpcm/login.go
+++ b/gpcm/login.go
@@ -4,11 +4,11 @@ import (
 	"crypto/md5"
 	"encoding/base64"
 	"encoding/hex"
-	"log"
 	"strconv"
 	"strings"
 	"wwfc/common"
 	"wwfc/database"
+	"wwfc/logging"
 )
 
 func generateResponse(gpcmChallenge, nasChallenge, authToken, clientChallenge string) string {
@@ -32,21 +32,30 @@ func generateProof(gpcmChallenge, nasChallenge, authToken, clientChallenge strin
 
 func (g *GameSpySession) login(command common.GameSpyCommand) {
 	if g.LoggedIn {
-		log.Fatalf("Attempt to login twice")
+		logging.Error(g.ModuleName, "Attempt to login twice")
+		g.replyError(ErrLogin)
+		return
+	}
+
+	if command.OtherValues["payload_ver"] != "1" {
+		g.replyError(GPError{
+			ErrorCode:   ErrLogin.ErrorCode,
+			ErrorString: "The payload version is invalid.",
+			Fatal:       true,
+		})
+		return
 	}
 
 	authToken := command.OtherValues["authtoken"]
 	challenge := database.GetChallenge(pool, ctx, authToken)
 	if challenge == "" {
-		// There was an error validating the pre-authentication.
-		g.replyError(ErrLoginBadPreAuth)
+		g.replyError(ErrLogin)
 		return
 	}
 
 	response := generateResponse(g.Challenge, challenge, authToken, command.OtherValues["challenge"])
 	if response != command.OtherValues["response"] {
-		// There was an error validating the pre-authentication.
-		g.replyError(ErrLoginBadPreAuth)
+		g.replyError(ErrLogin)
 		return
 	}