diff --git a/lib/datastore.js b/lib/datastore.js
index d35f81b..fbda4fe 100644
--- a/lib/datastore.js
+++ b/lib/datastore.js
@@ -58,7 +58,13 @@ Datastore.prototype.ensureIndex = function (options, cb) {
 
   options.datastore = this;
   this.indexes[options.fieldName] = new Index(options);
-  this.indexes[options.fieldName].insert(this.data);
+
+  try {
+    this.indexes[options.fieldName].insert(this.data);
+  } catch (e) {
+    delete this.indexes[options.fieldName];
+    return callback(e);
+  }
 
   return callback();
 };
diff --git a/lib/indexes.js b/lib/indexes.js
index 39bd5c8..bc19c97 100644
--- a/lib/indexes.js
+++ b/lib/indexes.js
@@ -39,6 +39,7 @@ function Index (options) {
 /**
  * Reset an index
  * @param {Document or Array of documents} newData Optional, data to initialize the index with
+ *                                                 If an error is thrown during insertion, the index is not modified
  */
 Index.prototype.reset = function (newData) {
   this.tree = new BinarySearchTree(this.treeOptions);
@@ -50,7 +51,7 @@ Index.prototype.reset = function (newData) {
 
 /**
  * Insert a new document in the index
- * If an array is passed, we insert all its elements
+ * If an array is passed, we insert all its elements (if one insertion fails the index is not modified)
  * O(log(n))
  */
 Index.prototype.insert = function (doc) {
diff --git a/test/db.test.js b/test/db.test.js
index 3f18589..1ee8935 100644
--- a/test/db.test.js
+++ b/test/db.test.js
@@ -1282,7 +1282,23 @@ describe('Database', function () {
         });
       });
 
-      it.skip('If a unique constraint is not respected, ensureIndex will return an error', function (done) {
+      it('If a unique constraint is not respected, ensureIndex will return an error', function (done) {
+        d.insert({ a: 1, b: 4 }, function () {
+          d.insert({ a: 2, b: 45 }, function () {
+            d.insert({ a: 1, b: 3 }, function () {
+              d.ensureIndex({ fieldName: 'b' }, function (err) {
+                assert.isUndefined(err);
+
+                d.ensureIndex({ fieldName: 'a', unique: true }, function (err) {
+                  err.errorType.should.equal('uniqueViolated');
+                  assert.deepEqual(Object.keys(d.indexes), ['b']);
+
+                  done();
+                });
+              });
+            });
+          });
+        });
       });
 
     });   // ==== End of 'ensureIndex' ==== //