C# - The Type Arguments For Method System.Linq.Enumerable.OrderBy Cannot Be Inferred From The Usage

I'm trying to follow the demo from this link to add a jqGrid to an MVC app.

I have a table named Companies that I'm trying to display in a grid. A Company simply contains an ID and a Name.

I'm running into an error in my controller function:

public JsonResult DynamicGridData(string sortIndex, string sortOrder, int page, int rows)
{
int pageIndex = Convert.ToInt32(page) - 1;
int pageSize = rows;
var companies = companiesRepository.Companies.OrderBy(sortIndex + " " + sortOrder).Skip(pageIndex * pageSize).Take(pageSize);
//Error here
...
}

I'm getting an error on the line that is calling OrderBy():

The type arguments for method 'System.Linq.Enumerable.OrderBy(System.Collections.Generic.IEnumerable, System.Func)' cannot be inferred from the usage. Try specifying the type arguments explicitly.

I really have no idea what the error means, and I haven't been able to find an explanation. I'm not sure what is causing this error on a simple OrderBy function.