diff --git a/source/api/actions/member_project.ts b/source/api/actions/member_project.ts
index 21cc165..faafec8 100644
--- a/source/api/actions/member_project.ts
+++ b/source/api/actions/member_project.ts
@@ -24,11 +24,15 @@ namespace _espe.api
 	{
 		register<
 			{
-				membership_number : string;
+				membership_number : (null | string);
 				name_real_value : string;
 				email_address_private : (null | string);
 			},
-			_espe.type.member_id
+			(
+				string
+				|
+				_espe.type.member_id
+			)
 		>(
 			rest_subject,
 			lib_plankton.http.enum_method.post,
@@ -71,25 +75,41 @@ namespace _espe.api
 						return Promise.reject(new Error("impossible"));
 					}
 					else {
-						const member_id : _espe.type.member_id = await _espe.service.member.project(
-							{
-								"membership_number": input.membership_number,
-								"name_real_value": input.name_real_value,
-								"email_address_private": (
-									("email_address_private" in input)
-									? (
-										(input.email_address_private !== "")
-										? input.email_address_private
+						if (
+							(! _espe.conf.get().settings.facultative_membership_number)
+							&&
+							(
+								(input.membership_number === null)
+								||
+								(input.membership_number === "")
+							)
+						) {
+							return Promise.resolve({
+								"status_code": 400,
+								"data": "membership number required"
+							});
+						}
+						else {
+							const member_id : _espe.type.member_id = await _espe.service.member.project(
+								{
+									"membership_number": input.membership_number,
+									"name_real_value": input.name_real_value,
+									"email_address_private": (
+										("email_address_private" in input)
+										? (
+											(input.email_address_private !== "")
+											? input.email_address_private
+											: null
+										)
 										: null
-									)
-									: null
-								),
-							}
-						);
-						return Promise.resolve({
-							"status_code": 201,
-							"data": member_id
-						});
+									),
+								}
+							);
+							return Promise.resolve({
+								"status_code": 201,
+								"data": member_id
+							});
+						}
 					}
 				}
 			}
diff --git a/source/conf.ts b/source/conf.ts
index 54fa333..5098ce4 100644
--- a/source/conf.ts
+++ b/source/conf.ts
@@ -97,6 +97,7 @@ namespace _espe.conf
 			target_domain : string;
 			frontend_url_base : (null | string);
 			prefix_for_nominal_email_addresses : string;
+			facultative_membership_number : boolean;
 			registration_email : {
 				subject : string;
 				body : string;
@@ -248,6 +249,7 @@ namespace _espe.conf
 					"target_domain": (node_settings["target_domain"] ?? "example.org"),
 					"frontend_url_base": (node_settings["frontend_url_base"] ?? null), // TODO: mandatory?
 					"prefix_for_nominal_email_addresses": (node_settings["prefix_for_nominal_email_addresses"] ?? "member-"),
+					"facultative_membership_number": (node_settings["facultative_membership_number"] ?? false),
 					"registration_email": {
 						"subject": ((node_settings["registration_email"] ?? {})["subject"] ?? "Registration"),
 						"body": ((node_settings["registration_email"] ?? {})["body"] ?? "URL: {{url}}"),
diff --git a/source/services/member.ts b/source/services/member.ts
index bcb513f..343b879 100644
--- a/source/services/member.ts
+++ b/source/services/member.ts
@@ -307,7 +307,7 @@ namespace _espe.service.member
 	 */
 	export async function project(
 		data : {
-			membership_number : string;
+			membership_number : (null | string);
 			name_real_value : string;
 			email_address_private : (null | string);
 		}