diff --git a/lxl-web/src/lib/types/search.ts b/lxl-web/src/lib/types/search.ts
index 1aeec90df..6256deb65 100644
--- a/lxl-web/src/lib/types/search.ts
+++ b/lxl-web/src/lib/types/search.ts
@@ -134,7 +134,7 @@ type MappingObj = { [key in SearchOperators]: SearchMapping[] | string | FramedD
 
 export interface SearchMapping extends MappingObj {
 	alias: string;
-	property?: ObjectProperty | DatatypeProperty | PropertyChainAxiom;
+	property?: ObjectProperty | DatatypeProperty | PropertyChainAxiom | InvalidProperty;
 	object?: FramedData;
 	up: { '@id': string };
 }
@@ -148,6 +148,11 @@ export interface DatatypeProperty {
 	'@id': string;
 }
 
+interface InvalidProperty {
+	'@type': '_Invalid';
+	label: string;
+}
+
 interface PropertyChainAxiom {
 	propertyChainAxiom: (ObjectProperty | DatatypeProperty)[];
 	label: string; // e.g. "instanceOf language"
diff --git a/lxl-web/src/lib/utils/search.ts b/lxl-web/src/lib/utils/search.ts
index 2925ba60e..139de61fd 100644
--- a/lxl-web/src/lib/utils/search.ts
+++ b/lxl-web/src/lib/utils/search.ts
@@ -26,6 +26,8 @@ import { type LocaleCode as LangCode } from '$lib/i18n/locales';
 import { bestImage, bestSize, toSecure } from '$lib/utils/auxd';
 import getAtPath from '$lib/utils/getAtPath';
 import { getUriSlug } from '$lib/utils/http';
+import { error } from '@sveltejs/kit';
+import { env } from '$env/dynamic/public';
 
 export async function asResult(
 	view: PartialCollectionView,
@@ -90,6 +92,16 @@ export function displayMappings(
 			const operator = _hasOperator(m);
 
 			if ('property' in m && operator) {
+				// Mock old behaviour for 'classic' search GUI, i.e show error page
+				// when encountering an invalid property in order to provide feedback.
+				// TODO remove this when Supersearch is fully implemented.
+				const useSuperSearch = env?.PUBLIC_USE_SUPERSEARCH === 'true';
+				if (!useSuperSearch && m.property?.['@type'] === '_Invalid') {
+					error(400, {
+						message: `Invalid query, please check the documentation. Unrecognized property alias: ${m.property?.label ?? ''}`
+					});
+				}
+
 				const property = m[operator] as FramedData;
 				return {
 					...(isObject(m.property) && { '@id': m.property['@id'] }),
@@ -102,7 +114,7 @@ export function displayMappings(
 							'No label', // lensandformat?
 					property:
 						m.property?.librisQueryCode ||
-						m.property?.['@id'].replace('https://id.kb.se/vocab/', '') ||
+						m.property?.['@id']?.replace('https://id.kb.se/vocab/', '') ||
 						'', //TODO replace with something better
 					operator,
 					...('up' in m && { up: replacePath(m.up as Link, usePath) })