diff --git a/docs/solid-typeindex-support.typeindexhelper.getfromtypeindex.md b/docs/solid-typeindex-support.typeindexhelper.getfromtypeindex.md
index d1febdc..3351cdc 100644
--- a/docs/solid-typeindex-support.typeindexhelper.getfromtypeindex.md
+++ b/docs/solid-typeindex-support.typeindexhelper.getfromtypeindex.md
@@ -17,7 +17,7 @@ static getFromTypeIndex(webId: string, rdfClass: string, fetch: any, isPrivate:
 |  Parameter | Type | Description |
 |  --- | --- | --- |
 |  webId | string | The user's WebID |
-|  rdfClass | string | The RDF class to retrieve instances for |
+|  rdfClass | string | The RDF class to retrieve instances for (it has to be a valid URL) |
 |  fetch | any | Authenticated fetch function |
 |  isPrivate | boolean | Whether the typeIndexe is private or public |
 
diff --git a/docs/solid-typeindex-support.typeindexhelper.registerintypeindex.md b/docs/solid-typeindex-support.typeindexhelper.registerintypeindex.md
index c263bd1..a4435f2 100644
--- a/docs/solid-typeindex-support.typeindexhelper.registerintypeindex.md
+++ b/docs/solid-typeindex-support.typeindexhelper.registerintypeindex.md
@@ -18,7 +18,7 @@ static registerInTypeIndex(webId: string, typeRegistrationTitle: string, rdfClas
 |  --- | --- | --- |
 |  webId | string | The WebID of the user |
 |  typeRegistrationTitle | string | The title to use for the type registration |
-|  rdfClass | string | The RDF class that this registration is for |
+|  rdfClass | string | The RDF class that this registration is for (it has to be a valid URL) |
 |  fetch | any | The authenticated fetch function |
 |  indexUrl | string | The URL of the index being registered |
 |  isPrivate | boolean | Whether to register in the private or public typeIndexe |
diff --git a/src/TypeIndexHelper.ts b/src/TypeIndexHelper.ts
index 995c225..fd1ba79 100644
--- a/src/TypeIndexHelper.ts
+++ b/src/TypeIndexHelper.ts
@@ -128,7 +128,7 @@ export class TypeIndexHelper {
      * Retrieves all instances of the given RDF class from the user's typeIndexe.
      *
      * @param webId - The user's WebID
-     * @param rdfClass - The RDF class to retrieve instances for
+     * @param rdfClass - The RDF class to retrieve instances for (it has to be a valid URL)
      * @param fetch - Authenticated fetch function
      * @param isPrivate - Whether the typeIndexe is private or public
      * @returns Promise resolving to an array of instance URLs
@@ -179,7 +179,7 @@ export class TypeIndexHelper {
      *
      * @param webId - The WebID of the user
      * @param typeRegistrationTitle - The title to use for the type registration
-     * @param rdfClass - The RDF class that this registration is for
+     * @param rdfClass - The RDF class that this registration is for (it has to be a valid URL)
      * @param fetch - The authenticated fetch function
      * @param indexUrl - The URL of the index being registered
      * @param isPrivate - Whether to register in the private or public typeIndexe