diff --git a/lib/src/binding_coap/coap_client.dart b/lib/src/binding_coap/coap_client.dart
index a551f2fe..cb1ad7d9 100644
--- a/lib/src/binding_coap/coap_client.dart
+++ b/lib/src/binding_coap/coap_client.dart
@@ -284,7 +284,7 @@ final class CoapClient extends ProtocolClient
 
     return AuthServerRequestCreationHint(
       authorizationServer:
-          aceSecurityScheme.as ?? creationHint?.authorizationServer,
+          aceSecurityScheme.as?.toString() ?? creationHint?.authorizationServer,
       scope: scope ?? creationHint?.scope,
       audience: aceSecurityScheme.audience ?? creationHint?.audience,
       clientNonce: creationHint?.clientNonce,
diff --git a/lib/src/core/definitions/security/ace_security_scheme.dart b/lib/src/core/definitions/security/ace_security_scheme.dart
index 4457d1ed..81d75fe4 100644
--- a/lib/src/core/definitions/security/ace_security_scheme.dart
+++ b/lib/src/core/definitions/security/ace_security_scheme.dart
@@ -40,7 +40,7 @@ final class AceSecurityScheme extends SecurityScheme {
     final jsonLdType = json.parseArrayField<String>("@type");
     final proxy = json.parseUriField("proxy", parsedFields);
 
-    final as = json.parseField<String>("ace:as", parsedFields);
+    final as = json.parseUriField("ace:as", parsedFields);
     final cnonce = json.parseField<bool>("ace:cnonce", parsedFields);
     final audience = json.parseField<String>("ace:audience", parsedFields);
     final scopes =
@@ -66,7 +66,7 @@ final class AceSecurityScheme extends SecurityScheme {
   String get scheme => aceSecuritySchemeName;
 
   /// URI of the authorization server.
-  final String? as;
+  final Uri? as;
 
   /// The intended audience for this [AceSecurityScheme].
   final String? audience;