diff --git a/src/Config.cs b/src/Config.cs
index 0b23d65..2b1044b 100644
--- a/src/Config.cs
+++ b/src/Config.cs
@@ -50,6 +50,12 @@ public class Config
             Console.WriteLine("Warning: Sites is deprecated, please use Instances instead");
         }
 
+        data.Tags ??= Array.Empty<string>();
+        if (data.MastodonPostgresConnectionString.HasValue() && data.Tags.Length > 0)
+        {
+            throw new Exception("You can't specify both MastodonPostgresConnectionString and Tags");
+        }
+
         Instance = new Config(importedPath, data.FakeRelayUrl, apiKey, data.MastodonPostgresConnectionString,
             data.Tags.ToImmutableArray(), data.GetImmutableSites());
     }
@@ -60,7 +66,7 @@ public class Config
         public string? FakeRelayApiKey { get; set; }
         public string? MastodonPostgresConnectionString { get; set; }
         public string[]? Instances { get; set; }
-        public string[] Tags { get; set; }
+        public string[]? Tags { get; set; }
         public InternalSiteData[]? Sites { get; set; }
 
         public ImmutableArray<SiteData> GetImmutableSites()
diff --git a/src/Extensions.cs b/src/Extensions.cs
new file mode 100644
index 0000000..9eed7c5
--- /dev/null
+++ b/src/Extensions.cs
@@ -0,0 +1,8 @@
+namespace GetMoarFediverse;
+
+public static class Extensions
+{
+    public static bool IsNullOrEmpty(this string s) => string.IsNullOrEmpty(s);
+
+    public static bool HasValue(this string s) => !s.IsNullOrEmpty();
+}