diff --git a/pagure/api/issue.py b/pagure/api/issue.py
index 1ef6edf..41ed130 100644
--- a/pagure/api/issue.py
+++ b/pagure/api/issue.py
@@ -825,7 +825,7 @@ def api_assign_issue(repo, issueid, username=None, namespace=None):
 
     form = pagure.forms.AssignIssueForm(csrf_enabled=False)
     if form.validate_on_submit():
-        assignee = form.assignee.data
+        assignee = form.assignee.data or None
         try:
             # New comment
             message = pagure.lib.add_issue_assignee(
diff --git a/tests/test_pagure_flask_api_issue.py b/tests/test_pagure_flask_api_issue.py
index dec2bb1..da78b94 100644
--- a/tests/test_pagure_flask_api_issue.py
+++ b/tests/test_pagure_flask_api_issue.py
@@ -1443,34 +1443,76 @@ class PagureFlaskApiIssuetests(tests.Modeltests):
         issue = pagure.lib.search_issues(self.session, repo, issueid=1)
         self.assertEqual(len(issue.comments), 0)
 
+        # No change
+        repo = pagure.lib.get_project(self.session, 'test')
+        issue = pagure.lib.search_issues(self.session, repo, issueid=1)
+        self.assertEqual(issue.status, 'Open')
+
         data = {
-            'title': 'test issue',
+            'assignee': 'pingou',
         }
 
-        # Incomplete request
+        # Valid request
         output = self.app.post(
             '/api/0/test/issue/1/assign', data=data, headers=headers)
-        self.assertEqual(output.status_code, 400)
+        self.assertEqual(output.status_code, 200)
         data = json.loads(output.data)
         self.assertDictEqual(
             data,
-            {
-              "error": "Invalid or incomplete input submited",
-              "error_code": "EINVALIDREQ",
-              "errors": {"assignee": ["This field is required."]}
-            }
+            {'message': 'Issue assigned'}
         )
 
-        # No change
+        # Un-assign
+        output = self.app.post(
+            '/api/0/test/issue/1/assign', data=data, headers=headers)
+        self.assertEqual(output.status_code, 200)
+        data = json.loads(output.data)
+        self.assertDictEqual(
+            data,
+            {'message': 'Assignee reset'}
+        )
         repo = pagure.lib.get_project(self.session, 'test')
         issue = pagure.lib.search_issues(self.session, repo, issueid=1)
-        self.assertEqual(issue.status, 'Open')
+        self.assertEqual(issue.assignee, None)
 
-        data = {
-            'assignee': 'pingou',
-        }
+        # Un-assign
+        data = {'assignee': None}
+        output = self.app.post(
+            '/api/0/test/issue/1/assign', data=data, headers=headers)
+        self.assertEqual(output.status_code, 200)
+        data = json.loads(output.data)
+        self.assertDictEqual(
+            data,
+            {'message': 'Nothing to change'}
+        )
+        repo = pagure.lib.get_project(self.session, 'test')
+        issue = pagure.lib.search_issues(self.session, repo, issueid=1)
+        self.assertEqual(issue.assignee, None)
 
-        # Valid request
+        # Re-assign for the rest of the tests
+        data = {'assignee': 'pingou'}
+        output = self.app.post(
+            '/api/0/test/issue/1/assign', data=data, headers=headers)
+        self.assertEqual(output.status_code, 200)
+        data = json.loads(output.data)
+        self.assertDictEqual(
+            data,
+            {'message': 'Issue assigned'}
+        )
+
+        # Un-assign
+        data = {'assignee': ''}
+        output = self.app.post(
+            '/api/0/test/issue/1/assign', data=data, headers=headers)
+        self.assertEqual(output.status_code, 200)
+        data = json.loads(output.data)
+        self.assertDictEqual(
+            data,
+            {'message': 'Assignee reset'}
+        )
+
+        # Re-assign for the rest of the tests
+        data = {'assignee': 'pingou'}
         output = self.app.post(
             '/api/0/test/issue/1/assign', data=data, headers=headers)
         self.assertEqual(output.status_code, 200)