diff --git a/auth/api/iam/api.go b/auth/api/iam/api.go
index 21a05464d1..9a0b4e3c1c 100644
--- a/auth/api/iam/api.go
+++ b/auth/api/iam/api.go
@@ -116,19 +116,27 @@ func (r Wrapper) HandleTokenRequest(ctx context.Context, request HandleTokenRequ
 		// Options:
 		// - OpenID4VCI
 		// - OpenID4VP, vp_token is sent in Token Response
-		panic("not implemented")
+		return nil, OAuth2Error{
+			Code:        UnsupportedGrantType,
+			Description: "not implemented yet",
+		}
 	case "vp_token":
 		// Options:
 		// - service-to-service vp_token flow
-		panic("not implemented")
+		return nil, OAuth2Error{
+			Code:        UnsupportedGrantType,
+			Description: "not implemented yet",
+		}
 	case "urn:ietf:params:oauth:grant-type:pre-authorized_code":
 		// Options:
 		// - OpenID4VCI
-		panic("not implemented")
+		return nil, OAuth2Error{
+			Code:        UnsupportedGrantType,
+			Description: "not implemented yet",
+		}
 	default:
 		return nil, OAuth2Error{
-			Code:        InvalidRequest,
-			Description: "unsupported grant_type",
+			Code: UnsupportedGrantType,
 		}
 	}
 }
diff --git a/auth/api/iam/error.go b/auth/api/iam/error.go
index a7e988905b..a70c005052 100644
--- a/auth/api/iam/error.go
+++ b/auth/api/iam/error.go
@@ -35,6 +35,8 @@ const (
 	// InvalidRequest is returned when the request is missing a required parameter, includes an invalid parameter value,
 	// includes a parameter more than once, or is otherwise malformed.
 	InvalidRequest ErrorCode = "invalid_request"
+	// UnsupportedGrantType is returned when the authorization grant type is not supported by the authorization server.
+	UnsupportedGrantType ErrorCode = "unsupported_grant_type"
 	// UnsupportedResponseType is returned when the authorization server does not support obtaining an authorization code using this method.
 	UnsupportedResponseType ErrorCode = "unsupported_response_type"
 	// ServerError is returned when the Authorization Server encounters an unexpected condition that prevents it from fulfilling the request.