Undefined function ctype_lower | WordPress.org

Hi. Any ideas how to resolve this error?

PHP Fatal error:  Uncaught Error: Call to undefined function FluentCrm\Framework\Support\ctype_lower() in /.../htdocs/wp/wp-content/plugins/fluent-crm/vendor/wpfluent/framework/src/WPFluent/Support/Str.php:402 
Stack trace: 
#0 /.../htdocs/wp/wp-content/plugins/fluent-crm/vendor/wpfluent/framework/src/WPFluent/Database/Orm/Model.php(3407): FluentCrm\Framework\Support\Str::snake('Photo') 
#1 /

My site has PHP 7.4.10.
WP 6.1.1
FluentCRM 2.6.51
Fluent Forms 4.3.22


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