diff --git a/demo/src/main/java/ch/dissem/bitmessage/demo/Application.java b/demo/src/main/java/ch/dissem/bitmessage/demo/Application.java
index 2de6fa1..05b6df2 100644
--- a/demo/src/main/java/ch/dissem/bitmessage/demo/Application.java
+++ b/demo/src/main/java/ch/dissem/bitmessage/demo/Application.java
@@ -212,7 +212,7 @@ public class Application {
             int i = 0;
             for (Plaintext message : messages) {
                 i++;
-                System.out.print(i + ") From: " + message.getFrom() + "; Subject: " + message.getSubject());
+                System.out.println(i + ") From: " + message.getFrom() + "; Subject: " + message.getSubject());
             }
             if (i == 0) {
                 System.out.println("You have no messages.");
diff --git a/domain/src/main/java/ch/dissem/bitmessage/entity/payload/Msg.java b/domain/src/main/java/ch/dissem/bitmessage/entity/payload/Msg.java
index 88026c8..f9e7c99 100644
--- a/domain/src/main/java/ch/dissem/bitmessage/entity/payload/Msg.java
+++ b/domain/src/main/java/ch/dissem/bitmessage/entity/payload/Msg.java
@@ -97,6 +97,7 @@ public class Msg extends ObjectPayload implements Encrypted {
 
     @Override
     public void write(OutputStream out) throws IOException {
+        if (encrypted == null) throw new IllegalStateException("Msg must be signed and encrypted before writing it.");
         encrypted.write(out);
     }
 }
diff --git a/domain/src/main/java/ch/dissem/bitmessage/exception/InsufficientProofOfWorkException.java b/domain/src/main/java/ch/dissem/bitmessage/exception/InsufficientProofOfWorkException.java
new file mode 100644
index 0000000..069bd79
--- /dev/null
+++ b/domain/src/main/java/ch/dissem/bitmessage/exception/InsufficientProofOfWorkException.java
@@ -0,0 +1,12 @@
+package ch.dissem.bitmessage.exception;
+
+import ch.dissem.bitmessage.utils.Strings;
+
+import java.io.IOException;
+import java.util.Arrays;
+
+public class InsufficientProofOfWorkException extends IOException {
+    public InsufficientProofOfWorkException(byte[] target, byte[] hash) {
+        super("Insufficient proof of work: " + Strings.hex(target) + " required, " + Strings.hex(Arrays.copyOfRange(hash, 0, 8)) + " achieved.");
+    }
+}
diff --git a/domain/src/main/java/ch/dissem/bitmessage/utils/Security.java b/domain/src/main/java/ch/dissem/bitmessage/utils/Security.java
index 978be35..db4faed 100644
--- a/domain/src/main/java/ch/dissem/bitmessage/utils/Security.java
+++ b/domain/src/main/java/ch/dissem/bitmessage/utils/Security.java
@@ -18,6 +18,7 @@ package ch.dissem.bitmessage.utils;
 
 import ch.dissem.bitmessage.entity.ObjectMessage;
 import ch.dissem.bitmessage.entity.payload.Pubkey;
+import ch.dissem.bitmessage.exception.InsufficientProofOfWorkException;
 import ch.dissem.bitmessage.factory.Factory;
 import ch.dissem.bitmessage.ports.ProofOfWorkEngine;
 import org.bouncycastle.asn1.x9.X9ECParameters;
@@ -106,13 +107,13 @@ public class Security {
     }
 
     /**
-     * @throws IOException if proof of work doesn't check out
+     * @throws InsufficientProofOfWorkException if proof of work doesn't check out
      */
     public static void checkProofOfWork(ObjectMessage object, long nonceTrialsPerByte, long extraBytes) throws IOException {
         byte[] target = getProofOfWorkTarget(object, nonceTrialsPerByte, extraBytes);
         byte[] value = Security.doubleSha512(object.getNonce(), getInitialHash(object));
         if (Bytes.lt(target, value, 8)) {
-            throw new IOException("Insufficient proof of work: " + Strings.hex(target) + " required, " + Strings.hex(Arrays.copyOfRange(value, 0, 8)) + " achieved.");
+            throw new InsufficientProofOfWorkException(target, value);
         }
     }