diff --git a/src/main/java/org/asamk/signal/commands/RegisterCommand.java b/src/main/java/org/asamk/signal/commands/RegisterCommand.java
index 971e4dd1..1b7e5ef9 100644
--- a/src/main/java/org/asamk/signal/commands/RegisterCommand.java
+++ b/src/main/java/org/asamk/signal/commands/RegisterCommand.java
@@ -27,7 +27,8 @@ public class RegisterCommand implements RegistrationCommand {
     @Override
     public void handleCommand(final Namespace ns, final RegistrationManager m) throws CommandException {
         final boolean voiceVerification = ns.getBoolean("voice");
-        final var captcha = ns.getString("captcha").replace("signalcaptcha://", "");
+        final var captchaString = ns.getString("captcha");
+        final var captcha = captchaString == null ? null : captchaString.replace("signalcaptcha://", "");
 
         try {
             m.register(voiceVerification, captcha);