diff --git a/java/src/main/java/org/whispersystems/libsignal/groups/GroupCipher.java b/java/src/main/java/org/whispersystems/libsignal/groups/GroupCipher.java
index d1d6bb60..687fd122 100644
--- a/java/src/main/java/org/whispersystems/libsignal/groups/GroupCipher.java
+++ b/java/src/main/java/org/whispersystems/libsignal/groups/GroupCipher.java
@@ -7,10 +7,12 @@
 
 import org.whispersystems.libsignal.DecryptionCallback;
 import org.whispersystems.libsignal.DuplicateMessageException;
+import org.whispersystems.libsignal.InvalidKeyException;
 import org.whispersystems.libsignal.InvalidKeyIdException;
 import org.whispersystems.libsignal.InvalidMessageException;
 import org.whispersystems.libsignal.LegacyMessageException;
 import org.whispersystems.libsignal.NoSessionException;
+import org.whispersystems.libsignal.ecc.ECPrivateKey;
 import org.whispersystems.libsignal.groups.ratchet.SenderChainKey;
 import org.whispersystems.libsignal.groups.ratchet.SenderMessageKey;
 import org.whispersystems.libsignal.groups.state.SenderKeyRecord;
@@ -56,19 +58,25 @@ public GroupCipher(SenderKeyStore senderKeyStore, SenderKeyName senderKeyId) {
    * @param paddedPlaintext The plaintext message bytes, optionally padded.
    * @return Ciphertext.
    * @throws NoSessionException
+   * @throws InvalidKeyException
    */
-  public byte[] encrypt(byte[] paddedPlaintext) throws NoSessionException {
+  public byte[] encrypt(byte[] paddedPlaintext) throws NoSessionException, InvalidKeyException {
     synchronized (LOCK) {
       try {
         SenderKeyRecord  record         = senderKeyStore.loadSenderKey(senderKeyId);
         SenderKeyState   senderKeyState = record.getSenderKeyState();
         SenderMessageKey senderKey      = senderKeyState.getSenderChainKey().getSenderMessageKey();
+        ECPrivateKey     signatureKey   = senderKeyState.getSigningKeyPrivate();
         byte[]           ciphertext     = getCipherText(senderKey.getIv(), senderKey.getCipherKey(), paddedPlaintext);
 
+        if (signatureKey == null) {
+          throw new InvalidKeyException("Session missing signature key!");
+        }
+
         SenderKeyMessage senderKeyMessage = new SenderKeyMessage(senderKeyState.getKeyId(),
                                                                  senderKey.getIteration(),
                                                                  ciphertext,
-                                                                 senderKeyState.getSigningKeyPrivate());
+                                                                 signatureKey);
 
         senderKeyState.setSenderChainKey(senderKeyState.getSenderChainKey().getNext());
 
diff --git a/java/src/main/java/org/whispersystems/libsignal/groups/state/SenderKeyState.java b/java/src/main/java/org/whispersystems/libsignal/groups/state/SenderKeyState.java
index 5824d8b4..30b980ae 100644
--- a/java/src/main/java/org/whispersystems/libsignal/groups/state/SenderKeyState.java
+++ b/java/src/main/java/org/whispersystems/libsignal/groups/state/SenderKeyState.java
@@ -98,8 +98,13 @@ public ECPublicKey getSigningKeyPublic() throws InvalidKeyException {
   }
 
   public ECPrivateKey getSigningKeyPrivate() {
-    return Curve.decodePrivatePoint(senderKeyStateStructure.getSenderSigningKey()
-                                                           .getPrivate().toByteArray());
+    if (senderKeyStateStructure.hasSenderSigningKey() &&
+        senderKeyStateStructure.getSenderSigningKey().hasPrivate()) {
+      return Curve.decodePrivatePoint(senderKeyStateStructure.getSenderSigningKey()
+                                                             .getPrivate().toByteArray());
+    } else {
+      return null;
+    }
   }
 
   public boolean hasSenderMessageKey(int iteration) {
diff --git a/tests/src/test/java/org/whispersystems/libsignal/groups/GroupCipherTest.java b/tests/src/test/java/org/whispersystems/libsignal/groups/GroupCipherTest.java
index 41082cd4..f96443ab 100644
--- a/tests/src/test/java/org/whispersystems/libsignal/groups/GroupCipherTest.java
+++ b/tests/src/test/java/org/whispersystems/libsignal/groups/GroupCipherTest.java
@@ -2,6 +2,7 @@
 
 import junit.framework.TestCase;
 
+import org.whispersystems.libsignal.InvalidKeyException;
 import org.whispersystems.libsignal.SignalProtocolAddress;
 import org.whispersystems.libsignal.DuplicateMessageException;
 import org.whispersystems.libsignal.InvalidMessageException;
@@ -22,7 +23,7 @@ public class GroupCipherTest extends TestCase {
   private static final SignalProtocolAddress SENDER_ADDRESS = new SignalProtocolAddress("+14150001111", 1);
   private static final SenderKeyName  GROUP_SENDER   = new SenderKeyName("nihilist history reading group", SENDER_ADDRESS);
 
-  public void testNoSession() throws InvalidMessageException, LegacyMessageException, NoSessionException, DuplicateMessageException {
+  public void testNoSession() throws InvalidMessageException, LegacyMessageException, NoSessionException, DuplicateMessageException, InvalidKeyException {
     InMemorySenderKeyStore aliceStore = new InMemorySenderKeyStore();
     InMemorySenderKeyStore bobStore   = new InMemorySenderKeyStore();
 
@@ -47,7 +48,7 @@ public void testNoSession() throws InvalidMessageException, LegacyMessageExcepti
   }
 
   public void testBasicEncryptDecrypt()
-      throws LegacyMessageException, DuplicateMessageException, InvalidMessageException, NoSessionException
+      throws LegacyMessageException, DuplicateMessageException, InvalidMessageException, NoSessionException, InvalidKeyException
   {
     InMemorySenderKeyStore aliceStore = new InMemorySenderKeyStore();
     InMemorySenderKeyStore bobStore   = new InMemorySenderKeyStore();
@@ -68,7 +69,7 @@ public void testBasicEncryptDecrypt()
     assertTrue(new String(plaintextFromAlice).equals("smert ze smert"));
   }
 
-  public void testLargeMessages() throws InvalidMessageException, LegacyMessageException, NoSessionException, DuplicateMessageException {
+  public void testLargeMessages() throws InvalidMessageException, LegacyMessageException, NoSessionException, DuplicateMessageException, InvalidKeyException {
     InMemorySenderKeyStore aliceStore = new InMemorySenderKeyStore();
     InMemorySenderKeyStore bobStore   = new InMemorySenderKeyStore();
 
@@ -92,7 +93,7 @@ public void testLargeMessages() throws InvalidMessageException, LegacyMessageExc
   }
 
   public void testBasicRatchet()
-      throws LegacyMessageException, DuplicateMessageException, InvalidMessageException, NoSessionException
+      throws LegacyMessageException, DuplicateMessageException, InvalidMessageException, NoSessionException, InvalidKeyException
   {
     InMemorySenderKeyStore aliceStore = new InMemorySenderKeyStore();
     InMemorySenderKeyStore bobStore   = new InMemorySenderKeyStore();
@@ -133,7 +134,7 @@ public void testBasicRatchet()
     assertTrue(new String(plaintextFromAlice3).equals("smert ze smert3"));
   }
 
-  public void testLateJoin() throws NoSessionException, InvalidMessageException, LegacyMessageException, DuplicateMessageException {
+  public void testLateJoin() throws NoSessionException, InvalidMessageException, LegacyMessageException, DuplicateMessageException, InvalidKeyException {
     InMemorySenderKeyStore aliceStore = new InMemorySenderKeyStore();
     InMemorySenderKeyStore bobStore   = new InMemorySenderKeyStore();
 
@@ -168,7 +169,7 @@ public void testLateJoin() throws NoSessionException, InvalidMessageException, L
 
 
   public void testOutOfOrder()
-      throws LegacyMessageException, DuplicateMessageException, InvalidMessageException, NoSessionException
+      throws LegacyMessageException, DuplicateMessageException, InvalidMessageException, NoSessionException, InvalidKeyException
   {
     InMemorySenderKeyStore aliceStore = new InMemorySenderKeyStore();
     InMemorySenderKeyStore bobStore   = new InMemorySenderKeyStore();
@@ -201,7 +202,7 @@ public void testOutOfOrder()
     }
   }
 
-  public void testEncryptNoSession() {
+  public void testEncryptNoSession() throws InvalidKeyException {
     InMemorySenderKeyStore aliceStore = new InMemorySenderKeyStore();
     GroupCipher aliceGroupCipher = new GroupCipher(aliceStore, new SenderKeyName("coolio groupio", new SignalProtocolAddress("+10002223333", 1)));
     try {
@@ -213,7 +214,7 @@ public void testEncryptNoSession() {
   }
 
 
-  public void testTooFarInFuture() throws DuplicateMessageException, InvalidMessageException, LegacyMessageException, NoSessionException {
+  public void testTooFarInFuture() throws DuplicateMessageException, InvalidMessageException, LegacyMessageException, NoSessionException, InvalidKeyException {
     InMemorySenderKeyStore aliceStore = new InMemorySenderKeyStore();
     InMemorySenderKeyStore bobStore   = new InMemorySenderKeyStore();
 
@@ -275,6 +276,27 @@ public void testMessageKeyLimit() throws Exception {
     }
   }
 
+  public void testInvalidSignatureKey()
+      throws LegacyMessageException, DuplicateMessageException, InvalidMessageException, NoSessionException
+  {
+    InMemorySenderKeyStore aliceStore = new InMemorySenderKeyStore();
+    InMemorySenderKeyStore bobStore   = new InMemorySenderKeyStore();
+
+    GroupSessionBuilder aliceSessionBuilder = new GroupSessionBuilder(aliceStore);
+    GroupSessionBuilder bobSessionBuilder   = new GroupSessionBuilder(bobStore);
+
+    GroupCipher bobGroupCipher   = new GroupCipher(bobStore, GROUP_SENDER);
+
+    SenderKeyDistributionMessage sentAliceDistributionMessage     = aliceSessionBuilder.create(GROUP_SENDER);
+    SenderKeyDistributionMessage receivedAliceDistributionMessage = new SenderKeyDistributionMessage(sentAliceDistributionMessage.serialize());
+    bobSessionBuilder.process(GROUP_SENDER, receivedAliceDistributionMessage);
+
+    try {
+      bobGroupCipher.encrypt("smert ze smert".getBytes());
+    } catch (InvalidKeyException e) {
+      // good
+    }
+  }
 
   private int randomInt() {
     try {