diff --git a/lib/contracts/reimbursement.js b/lib/contracts/reimbursement.js
index 585037a..ff027cb 100644
--- a/lib/contracts/reimbursement.js
+++ b/lib/contracts/reimbursement.js
@@ -25,10 +25,21 @@ class Reimbursement extends Record {
     const token = attrs.token;
     const contributorId = attrs.contributorId;
     const expenses = attrs.expenses.map( e => new ExpenseSerializer(e) );
+    let errorMessage;
 
-    if (!amount > 0 || !token || token === '' || !contributorId || contributorId === '' || !expenses.length > 0) {
-      return Promise.reject(new Error('Invalid data. amount, token, expenses is required.'));
+    if (typeof amount !== 'number' || amount <= 0) {
+      errorMessage = 'Invalid data: amount must be a positive number.';
     }
+    if (!token || token === '') {
+      errorMessage = 'Invalid data: token must be a token address.';
+    }
+    if (!contributorId || contributorId === '') {
+      errorMessage = 'Invalid data: contributorId is required.';
+    }
+    if (expenses.length === 0) {
+      errorMessage = 'Invalid data: at least one expense item is required.';
+    }
+    if (errorMessage) { return Promise.reject(new Error(errorMessage)); }
 
     return Promise.all(expenses.map(e => e.validate()))
       .then(() => {
@@ -39,7 +50,7 @@ class Reimbursement extends Record {
             const reimbursement = [
               amount,
               token,
-              contributorId,
+              parseInt(contributorId),
               ipfsHashAttr.hashDigest,
               ipfsHashAttr.hashFunction,
               ipfsHashAttr.hashSize,