diff --git a/core/src/main/java/com/google/bitcoin/core/Wallet.java b/core/src/main/java/com/google/bitcoin/core/Wallet.java
index 38a2e309..da6b1cbd 100644
--- a/core/src/main/java/com/google/bitcoin/core/Wallet.java
+++ b/core/src/main/java/com/google/bitcoin/core/Wallet.java
@@ -17,7 +17,6 @@
 package com.google.bitcoin.core;
 
 import com.google.bitcoin.core.TransactionConfidence.ConfidenceType;
-import com.google.bitcoin.wallet.WalletTransaction.Pool;
 import com.google.bitcoin.crypto.KeyCrypter;
 import com.google.bitcoin.crypto.KeyCrypterException;
 import com.google.bitcoin.crypto.KeyCrypterScrypt;
@@ -29,6 +28,7 @@ import com.google.bitcoin.store.WalletProtobufSerializer;
 import com.google.bitcoin.utils.ListenerRegistration;
 import com.google.bitcoin.utils.Threading;
 import com.google.bitcoin.wallet.*;
+import com.google.bitcoin.wallet.WalletTransaction.Pool;
 import com.google.common.collect.*;
 import com.google.common.primitives.Ints;
 import com.google.common.util.concurrent.FutureCallback;
@@ -190,14 +190,6 @@ public class Wallet implements Serializable, BlockChainListener, PeerFilterProvi
      * see loadFromFile.
      */
     public Wallet(NetworkParameters params) {
-        this(params, null);
-    }
-
-    /**
-     * Create a wallet with a keyCrypter to use in encrypting and decrypting keys.
-     */
-    public Wallet(NetworkParameters params, KeyCrypter keyCrypter) {
-        this.keyCrypter = keyCrypter;
         this.params = checkNotNull(params);
         keychain = new ArrayList<ECKey>();
         watchedScripts = Sets.newHashSet();
@@ -212,6 +204,14 @@ public class Wallet implements Serializable, BlockChainListener, PeerFilterProvi
         createTransientState();
     }
 
+    /**
+     * Create a wallet with a keyCrypter to use in encrypting and decrypting keys.
+     */
+    public Wallet(NetworkParameters params, KeyCrypter keyCrypter) {
+        this(params);
+        this.keyCrypter = checkNotNull(keyCrypter);
+    }
+
     private void createTransientState() {
         ignoreNextNewBlock = new HashSet<Sha256Hash>();
         txConfidenceListener = new TransactionConfidence.Listener() {
@@ -1593,7 +1593,9 @@ public class Wallet implements Serializable, BlockChainListener, PeerFilterProvi
          */
         public static SendRequest to(Address destination, BigInteger value) {
             SendRequest req = new SendRequest();
-            req.tx = new Transaction(destination.getParameters());
+            final NetworkParameters parameters = destination.getParameters();
+            checkNotNull(parameters, "Address is for an unknown network");
+            req.tx = new Transaction(parameters);
             req.tx.addOutput(value, destination);
             return req;
         }
@@ -1622,7 +1624,9 @@ public class Wallet implements Serializable, BlockChainListener, PeerFilterProvi
 
         public static SendRequest emptyWallet(Address destination) {
             SendRequest req = new SendRequest();
-            req.tx = new Transaction(destination.getParameters());
+            final NetworkParameters parameters = destination.getParameters();
+            checkNotNull(parameters, "Address is for an unknown network");
+            req.tx = new Transaction(parameters);
             req.tx.addOutput(BigInteger.ZERO, destination);
             req.emptyWallet = true;
             return req;
@@ -1768,7 +1772,6 @@ public class Wallet implements Serializable, BlockChainListener, PeerFilterProvi
      *
      * @return The {@link Transaction} that was created or null if there was insufficient balance to send the coins.
      * @throws InsufficientMoneyException if the request could not be completed due to not enough balance.
-     * @throws IOException if there was a problem broadcasting the transaction
      */
     public Transaction sendCoins(Peer peer, SendRequest request) throws InsufficientMoneyException {
         Transaction tx = sendCoinsOffline(request);
@@ -2099,6 +2102,7 @@ public class Wallet implements Serializable, BlockChainListener, PeerFilterProvi
      *
      * @return ECKey object or null if no such key was found.
      */
+    @Nullable
     public ECKey findKeyFromPubHash(byte[] pubkeyHash) {
         lock.lock();
         try {
@@ -2142,6 +2146,7 @@ public class Wallet implements Serializable, BlockChainListener, PeerFilterProvi
      * Locates a keypair from the keychain given the raw public key bytes.
      * @return ECKey or null if no such key was found.
      */
+    @Nullable
     public ECKey findKeyFromPubKey(byte[] pubkey) {
         lock.lock();
         try {
@@ -2334,7 +2339,7 @@ public class Wallet implements Serializable, BlockChainListener, PeerFilterProvi
     }
 
     private void toStringHelper(StringBuilder builder, Map<Sha256Hash, Transaction> transactionMap,
-                                AbstractBlockChain chain) {
+                                @Nullable AbstractBlockChain chain) {
         checkState(lock.isHeldByCurrentThread());
         for (Transaction tx : transactionMap.values()) {
             try {
@@ -2811,11 +2816,7 @@ public class Wallet implements Serializable, BlockChainListener, PeerFilterProvi
     public boolean checkPassword(CharSequence password) {
         lock.lock();
         try {
-            if (keyCrypter == null) {
-                // The password cannot decrypt anything as the keyCrypter is null.
-                return false;
-            }
-            return checkAESKey(keyCrypter.deriveKey(checkNotNull(password)));
+            return keyCrypter != null && checkAESKey(keyCrypter.deriveKey(checkNotNull(password)));
         } finally {
             lock.unlock();
         }
@@ -3513,15 +3514,16 @@ public class Wallet implements Serializable, BlockChainListener, PeerFilterProvi
             try {
                 if (output.getScriptPubKey().isSentToAddress()) {
                     // Send-to-address spends usually take maximum pubkey.length (as it may be compressed or not) + 75 bytes
-                    size += findKeyFromPubHash(output.getScriptPubKey().getPubKeyHash()).getPubKey().length + 75;
+                    final ECKey key = findKeyFromPubHash(output.getScriptPubKey().getPubKeyHash());
+                    size += checkNotNull(key, "Coin selection includes unspendable outputs").getPubKey().length + 75;
                 } else if (output.getScriptPubKey().isSentToRawPubKey())
                     size += 74; // Send-to-pubkey spends usually take maximum 74 bytes to spend
                 else
-                    throw new RuntimeException("Unknown output type returned in coin selection");
+                    throw new IllegalStateException("Unknown output type returned in coin selection");
             } catch (ScriptException e) {
                 // If this happens it means an output script in a wallet tx could not be understood. That should never
                 // happen, if it does it means the wallet has got into an inconsistent state.
-                throw new RuntimeException(e);
+                throw new IllegalStateException(e);
             }
         }
         return size;
@@ -3636,6 +3638,7 @@ public class Wallet implements Serializable, BlockChainListener, PeerFilterProvi
         } while (tx != null && tx.getInputs().size() == KeyTimeCoinSelector.MAX_SIMULTANEOUS_INPUTS);
     }
 
+    @Nullable
     private Transaction rekeyOneBatch(long keyRotationTimestamp, final TransactionBroadcaster broadcaster) {
         final Transaction rekeyTx;
 
@@ -3652,7 +3655,8 @@ public class Wallet implements Serializable, BlockChainListener, PeerFilterProvi
                     safeKey = key;
                 }
             }
-            if (!haveRotatingKeys) return null;
+            if (!haveRotatingKeys)
+                return null;
             if (safeKey == null) {
                 log.warn("Key rotation requested but no keys newer than the timestamp are available.");
                 return null;