diff --git a/src/Cellm/AddIn/ArgumentParser.cs b/src/Cellm/AddIn/ArgumentParser.cs
index 8684adf..168c463 100644
--- a/src/Cellm/AddIn/ArgumentParser.cs
+++ b/src/Cellm/AddIn/ArgumentParser.cs
@@ -3,11 +3,10 @@
 using Cellm.Models.Providers;
 using ExcelDna.Integration;
 using Microsoft.Extensions.Configuration;
-using Microsoft.Office.Interop.Excel;
 
 namespace Cellm.AddIn;
 
-public record Arguments(string Provider, string Model, string Context, string Instructions, double Temperature);
+public record Arguments(Provider Provider, string Model, string Context, string Instructions, double Temperature);
 
 public class ArgumentParser
 {
@@ -71,11 +70,16 @@ public ArgumentParser AddTemperature(object temperature)
 
     public Arguments Parse()
     {
-        var provider = _provider ?? _configuration
+        var providerAsString = _provider ?? _configuration
             .GetSection(nameof(ProviderConfiguration))
             .GetValue<string>(nameof(ProviderConfiguration.DefaultProvider))
             ?? throw new ArgumentException(nameof(ProviderConfiguration.DefaultProvider));
 
+        if (!Enum.TryParse<Provider>(providerAsString, true, out var provider))
+        {
+            throw new ArgumentException($"Unsupported default provider: {providerAsString}");
+        }
+
         var model = _model ?? _configuration
             .GetSection($"{provider}Configuration")
             .GetValue<string>(nameof(IProviderConfiguration.DefaultModel))
@@ -147,9 +151,9 @@ private static string ParseCells(ExcelReference reference)
     {
         try
         {
-            var app = (Application)ExcelDnaUtil.Application;
+            var app = ExcelDnaUtil.Application;
             var sheetName = (string)XlCall.Excel(XlCall.xlSheetNm, reference);
-            sheetName = sheetName[(sheetName.LastIndexOf("]") + 1)..];
+            sheetName = sheetName[(sheetName.LastIndexOf(']') + 1)..];
             var worksheet = app.Sheets[sheetName];
 
             var tableBuilder = new StringBuilder();