diff --git a/src/fetch_series.m b/src/fetch_series.m
index 511f9ffabc0daf1135818eb646264b2f55f70fed..83012d1787785c8e7563f7a7b4e4af6759186cd5 100644
--- a/src/fetch_series.m
+++ b/src/fetch_series.m
@@ -56,7 +56,7 @@ default_api_base_url = 'https://api.db.nomics.world/v22/';
 default_editor_base_url = 'https://editor.nomics.world/api/v1/';
 
 p = inputParser;
-validStringInput = @(x) isstring(x) || ischar(x) || iscellstr(x);
+validStringInput = @(x) ischar(x) || iscellstr(x);
 p.addParameter('provider_code', '', validStringInput);
 p.addParameter('dataset_code', '', validStringInput);
 p.addParameter('series_code', '', validStringInput); 
diff --git a/src/fetch_series_by_api_link.m b/src/fetch_series_by_api_link.m
index 14768d1dcb5f9e83c5575338004e5b0894cca476..88c3f24d1fa936104885b4e3374ad737e62224b1 100644
--- a/src/fetch_series_by_api_link.m
+++ b/src/fetch_series_by_api_link.m
@@ -40,7 +40,7 @@ function df = fetch_series_by_api_link(api_link, varargin) % --*-- Unitary tests
 default_editor_base_url = 'https://editor.nomics.world/api/v1/';
 
 p = inputParser;
-validStringInput = @(x) isstring(x) || ischar(x) || iscellstr(x);
+validStringInput = @(x) ischar(x) || iscellstr(x);
 p.addRequired('api_link');
 p.addOptional('dbnomics_filters', '',@ischar);
 p.addOptional('max_nb_series', NaN, @isnumeric);