diff --git a/src/github/index.js b/src/github/index.js
index 6c0abc3..036e376 100644
--- a/src/github/index.js
+++ b/src/github/index.js
@@ -140,6 +140,14 @@ function getBlameForCommitFile (resource) {
 }
 
 function assignUsersToResource (resource, assignees) {
+  assignees = assignees || [];
+
+  for(var i = 0; i < assignees.length; i++) {
+    if (typeof assignees[i] !== 'string') {
+      throw Error('Assignees must be specified as strings');
+    }
+  }
+
   return github.issues.addAssigneesToIssue({
     'owner': resource.owner,
     'repo': resource.repo,
diff --git a/test/review-test.js b/test/review-test.js
index ef2275f..2c9e1be 100644
--- a/test/review-test.js
+++ b/test/review-test.js
@@ -270,7 +270,29 @@ describe('(unit)', function () {
     });
 
     it('#getBlameForCommitFile');
-    it('#assignUsersToResource');
+
+    describe('#assignUsersToResource', function () {
+      it('works correctly', function () {
+        ghapi.post('/repos/OWNER/REPO/issues/1/assignees').reply(200);
+
+        return github.assignUsersToResource({
+          'owner': 'OWNER',
+          'repo': 'REPO',
+          'number': '1'
+        }, ['test']);
+      });
+
+      it('fails with non-text assignees', function () {
+        (function () {
+          github.assignUsersToResource({
+            'owner': 'OWNER',
+            'repo': 'REPO',
+            'number': '1'
+          }, [{}]);
+        }).should.throw(Error, 'Assignees must be specified as strings');
+      });
+    });
+
     it('#postPullRequestComment');
 
     it('#getRepoFile', function () {