diff --git a/sample-programs/names.scm b/sample-programs/names.scm
index 0c2d2f8dc..79f868a54 100644
--- a/sample-programs/names.scm
+++ b/sample-programs/names.scm
@@ -11,38 +11,41 @@
 (define-public (preorder 
                 (name-hash (buff 20))
                 (name-price int))
-  (if (contract-call! tokens token-transfer
-                      burn-address name-price)
-      (insert-entry! preorder-map
+  (if (is-ok? (contract-call! tokens token-transfer
+                burn-address name-price))
+      (begin (insert-entry! preorder-map
                      (tuple (name-hash name-hash))
                      (tuple (paid name-price)
                             (buyer tx-sender)))
-      'false))
+             (ok 0))
+      (err "token payment failed.")))
 
 (define-public (register 
                 (recipient-principal principal)
                 (name int)
                 (salt int))
   (let ((preorder-entry
-         (fetch-entry preorder-map
-                      (tuple (name-hash (hash160 (xor name salt))))))
+         (expects! ;; name _must_ have been preordered.
+           (fetch-entry preorder-map
+                      (tuple (name-hash (hash160 (xor name salt)))))
+           (err "no preorder found")))
         (name-entry 
          (fetch-entry name-map (tuple (name name)))))
     (if (and
-         ;; must be preordered
-         (not (eq? preorder-entry 'null))
          ;; name shouldn't *already* exist
-         (eq? name-entry 'null)
+         (is-none? name-entry)
          ;; preorder must have paid enough
          (<= (price-function name) 
              (get paid preorder-entry))
          ;; preorder must have been the current principal
          (eq? tx-sender
               (get buyer preorder-entry)))
-        (and
-         (insert-entry! name-map
+        (if (and
+              (insert-entry! name-map
                         (tuple (name name))
                         (tuple (owner recipient-principal)))
-         (delete-entry! preorder-map
+              (delete-entry! preorder-map
                         (tuple (name-hash (hash160 (xor name salt))))))
-        'false)))
+            (ok 0)
+            (err "failed to insert new name entry"))
+        (err "invalid name register"))))
diff --git a/sample-programs/tokens.scm b/sample-programs/tokens.scm
index 0a50a6d44..3a1ee293c 100644
--- a/sample-programs/tokens.scm
+++ b/sample-programs/tokens.scm
@@ -1,21 +1,20 @@
 (define-map tokens ((account principal)) ((balance int)))
 (define (get-balance (account principal))
-  (let ((balance
-         (get balance (fetch-entry tokens (tuple (account account))))))
-    (if (eq? balance 'null) 0 balance)))
+  (default-to 0 (get balance (fetch-entry tokens (tuple (account account))))))
 
 (define (token-credit! (account principal) (tokens int))
   (if (<= tokens 0)
-      'false
+      (err "must move positive balance")
       (let ((current-amount (get-balance account)))
         (begin
           (set-entry! tokens (tuple (account account))
                       (tuple (balance (+ tokens current-amount))))
-          'true))))
+          (ok tokens)))))
+
 (define-public (token-transfer (to principal) (amount int))
   (let ((balance (get-balance tx-sender)))
     (if (or (> amount balance) (<= amount 0))
-        'false
+        (err "must transfer positive balance and possess funds")
         (begin
           (set-entry! tokens (tuple (account tx-sender))
                       (tuple (balance (- balance amount))))
@@ -23,10 +22,7 @@
 
 (define-public (mint! (amount int))
    (let ((balance (get-balance tx-sender)))
-     (begin (set-entry! tokens (tuple (account tx-sender))
-                        (tuple (balance (+ balance amount))))
-            'true)))
+     (token-credit! tx-sender amount)))
 
-(begin (token-credit! 'SZ2J6ZY48GV1EZ5V2V5RB9MP66SW86PYKKQ9H6DPR 10000)
-       (token-credit! 'SM2J6ZY48GV1EZ5V2V5RB9MP66SW86PYKKQVX8X0G 300)
-       'null)
+(token-credit! 'SZ2J6ZY48GV1EZ5V2V5RB9MP66SW86PYKKQ9H6DPR 10000)
+(token-credit! 'SM2J6ZY48GV1EZ5V2V5RB9MP66SW86PYKKQVX8X0G 300)
diff --git a/src/clarity.rs b/src/clarity.rs
index 339e55b91..61a61ae29 100644
--- a/src/clarity.rs
+++ b/src/clarity.rs
@@ -409,12 +409,12 @@ pub fn invoke_command(invoked_by: &str, args: &[String]) {
             };
             match result {
                 Ok(x) => {
-                    if let Value::Bool(x) = x {
+                    if let Value::Response(data) = x {
                         vm_env.commit();
-                        if x {
-                            println!("Transaction executed and committed.");
+                        if data.committed {
+                            println!("Transaction executed and committed. Returned: {}", data.data);
                         } else {
-                            println!("Aborted: Transaction returned false.");
+                            println!("Aborted: {}", data.data);
                         }
                     } else {
                         panic!(format!("Expected a bool result from transaction. Found: {}", x));
diff --git a/src/vm/database/sqlite.rs b/src/vm/database/sqlite.rs
index 6e5c7fe84..d62bc06db 100644
--- a/src/vm/database/sqlite.rs
+++ b/src/vm/database/sqlite.rs
@@ -38,9 +38,10 @@ impl ContractDatabaseConnection {
         contract_db.execute("CREATE TABLE IF NOT EXISTS maps_table
                       (map_identifier INTEGER PRIMARY KEY AUTOINCREMENT,
                        contract_name TEXT NOT NULL,
-                       map_name TEXT UNIQUE NOT NULL,
+                       map_name TEXT NOT NULL,
                        key_type TEXT NOT NULL,
-                       value_type TEXT NOT NULL)",
+                       value_type TEXT NOT NULL,
+                       UNIQUE(contract_name, map_name))",
                             NO_PARAMS);
         contract_db.execute("CREATE TABLE IF NOT EXISTS data_table
                       (data_identifier INTEGER PRIMARY KEY AUTOINCREMENT,