diff --git a/src/acl/access-groups.ts b/src/acl/access-groups.ts
index bf78337f..63f4dd7a 100644
--- a/src/acl/access-groups.ts
+++ b/src/acl/access-groups.ts
@@ -258,7 +258,15 @@ export class AccessGroups {
       }
       return this.handleDroppedUri(uri, combo, true)
     } else if (!agent) {
-      const error = `   Error: Drop fails to drop appropriate thing! ${uri}`
+      const detectedTypes = Object.keys(this.store.findTypeURIs(thing))
+      const typeDetails = detectedTypes.length > 0
+        ? `Detected RDF types: ${detectedTypes.join(', ')}`
+        : 'No RDF type was detected for this URI.'
+      const error =
+        `Error: Failed to add access target: ${uri} is not a recognized ACL target type.` +
+        ` Expected one of: vcard:WebID, vcard:Group, foaf:Person, foaf:Agent, solid:AppProvider, solid:AppProviderClass, or recognized ACL classes.` +
+        ' Hint: try dropping a WebID profile URI, a vcard:Group URI, or a web app origin.' +
+        typeDetails
       debug.error(error)
       return Promise.reject(new Error(error))
     }