diff --git a/schemas/course-credential.json b/schemas/course-credential.json
index 6efce05..d576472 100644
--- a/schemas/course-credential.json
+++ b/schemas/course-credential.json
@@ -117,7 +117,7 @@
                             "description": "The number of (ECTS) credits awarded for the course"
                         },
                         "dateOfAssessment": {
-                            "type": "string",
+                            "type": ["string", "null"],
                             "description": "The date of assessment",
                             "format": "date"
                         },
@@ -142,4 +142,4 @@
             }
         }
     ]
-}
\ No newline at end of file
+}
diff --git a/schemas/federation-membership.json b/schemas/federation-membership.json
index 88fd856..316f57a 100644
--- a/schemas/federation-membership.json
+++ b/schemas/federation-membership.json
@@ -100,7 +100,7 @@
               "type": "string"
             },
             "website": {
-              "type": "string",
+              "type": ["string", "null"],
               "format": "uri"
             },
             "evidence": {
@@ -129,4 +129,4 @@
       }
     }
   ]
-}
\ No newline at end of file
+}
diff --git a/schemas/membership-card.json b/schemas/membership-card.json
index 2a4a793..001fb4a 100644
--- a/schemas/membership-card.json
+++ b/schemas/membership-card.json
@@ -52,7 +52,7 @@
               "minLength":  1
             },
             "affiliatedUntil": {
-              "type": "string",
+              "type": ["string", "null"],
               "format": "date"
             },
             "typeOfPerson": {
@@ -100,4 +100,4 @@
       }
     }
   ]
-}
\ No newline at end of file
+}