LINQ subquery in azure function app returns error message

I have an azure function for getting Mappings from a azure cosmos db document. However, it fails when trying to run a query within a query.

        private static string GetKey(IEnumerable<Server> servers, int serverId)
            => servers.Where(s => s.ServerId == serverId).ToList().First().Key;
        

        public static IEnumerable<Mapping> GetMappings(DocumentClient documentClient)
            => GetSitesDocumentQuery(documentClient)
                .SelectMany(site => site.Terminals
                    .Select(terminal => new Mapping
                    {
                        Key = GetKey(site.Servers, terminal.ServerId),
                        // [...]
                    })
                )
                .ToList();

This fails with the following error message:

System.Private.CoreLib: Exception while executing function:
 ProcessMessage. System.Private.CoreLib: One or more errors occurred. 
 (Method 'GetKey' is not supported., Windows/10.0.19042 documentdb-netcore-sdk/2.12.0). 
Microsoft.Azure.DocumentDB.Core: Method 'GetKey' is not supported., Windows/10.0.19042 documentdb-netcore-sdk/2.12.0.

They're both members of the same class.

If I inline the query

        public static IEnumerable<Mapping> GetMappings(DocumentClient documentClient)
            => GetSitesDocumentQuery(documentClient)
                .SelectMany(site => site.Terminals
                    .Select(terminal => new Mapping
                    {
                        Key = site.Servers.Where(s => s.ServerId == serverId).AsEnumerable().First().Key,
                        // [...]
                    })
                )
                .ToList();

I get the same error message except now it claims First() is not supported.