diff --git a/services/Directory/FilterLists.Directory.Api.Contracts/Models/ListDetailsVm.cs b/services/Directory/FilterLists.Directory.Api.Contracts/Models/ListDetailsVm.cs
index ba0bee30f..c5218f8be 100644
--- a/services/Directory/FilterLists.Directory.Api.Contracts/Models/ListDetailsVm.cs
+++ b/services/Directory/FilterLists.Directory.Api.Contracts/Models/ListDetailsVm.cs
@@ -6,10 +6,10 @@ public record ListDetailsVm
     public string Name { get; init; } = null!;
     public string? Description { get; init; }
     public int? LicenseId { get; init; }
-    public IEnumerable<int>? SyntaxIds { get; init; }
-    public IEnumerable<string>? Iso6391s { get; init; }
-    public IEnumerable<int>? TagIds { get; init; }
-    public IEnumerable<ViewUrlVm>? ViewUrls { get; init; }
+    public IReadOnlyCollection<int> SyntaxIds { get; init; } = new HashSet<int>();
+    public IReadOnlyCollection<string> Iso6391s { get; init; } = new HashSet<string>();
+    public IReadOnlyCollection<int> TagIds { get; init; } = new HashSet<int>();
+    public IReadOnlyCollection<ViewUrlVm> ViewUrls { get; init; } = new HashSet<ViewUrlVm>();
     public Uri? HomeUrl { get; init; }
     public Uri? OnionUrl { get; init; }
     public Uri? PolicyUrl { get; init; }
@@ -19,13 +19,13 @@ public record ListDetailsVm
     public Uri? ChatUrl { get; init; }
     public string? EmailAddress { get; init; }
     public Uri? DonateUrl { get; init; }
-    public IEnumerable<int>? MaintainerIds { get; init; }
-    public IEnumerable<int>? UpstreamFilterListIds { get; init; }
-    public IEnumerable<int>? ForkFilterListIds { get; init; }
-    public IEnumerable<int>? IncludedInFilterListIds { get; init; }
-    public IEnumerable<int>? IncludesFilterListIds { get; init; }
-    public IEnumerable<int>? DependencyFilterListIds { get; init; }
-    public IEnumerable<int>? DependentFilterListIds { get; init; }
+    public IReadOnlyCollection<int> MaintainerIds { get; init; } = new HashSet<int>();
+    public IReadOnlyCollection<int> UpstreamFilterListIds { get; init; } = new HashSet<int>();
+    public IReadOnlyCollection<int> ForkFilterListIds { get; init; } = new HashSet<int>();
+    public IReadOnlyCollection<int> IncludedInFilterListIds { get; init; } = new HashSet<int>();
+    public IReadOnlyCollection<int> IncludesFilterListIds { get; init; } = new HashSet<int>();
+    public IReadOnlyCollection<int> DependencyFilterListIds { get; init; } = new HashSet<int>();
+    public IReadOnlyCollection<int> DependentFilterListIds { get; init; } = new HashSet<int>();
 
     public record ViewUrlVm
     {
diff --git a/services/Directory/FilterLists.Directory.Api.Contracts/Models/ListVm.cs b/services/Directory/FilterLists.Directory.Api.Contracts/Models/ListVm.cs
index c07ed4a64..e33e31112 100644
--- a/services/Directory/FilterLists.Directory.Api.Contracts/Models/ListVm.cs
+++ b/services/Directory/FilterLists.Directory.Api.Contracts/Models/ListVm.cs
@@ -6,9 +6,9 @@ public record ListVm
     public string Name { get; init; } = null!;
     public string? Description { get; init; }
     public int? LicenseId { get; init; }
-    public IEnumerable<int>? SyntaxIds { get; init; }
-    public IEnumerable<string>? Iso6391s { get; init; }
-    public IEnumerable<int>? TagIds { get; init; }
+    public IReadOnlyCollection<int> SyntaxIds { get; init; } = new HashSet<int>();
+    public IReadOnlyCollection<string> Iso6391s { get; init; } = new HashSet<string>();
+    public IReadOnlyCollection<int> TagIds { get; init; } = new HashSet<int>();
     public Uri? PrimaryViewUrl { get; init; }
-    public IEnumerable<int>? MaintainerIds { get; init; }
+    public IReadOnlyCollection<int> MaintainerIds { get; init; } = new HashSet<int>();
 }
diff --git a/services/Directory/FilterLists.Directory.Application/Queries/GetLanguages.cs b/services/Directory/FilterLists.Directory.Application/Queries/GetLanguages.cs
index 8cd107658..fb6ed5179 100644
--- a/services/Directory/FilterLists.Directory.Application/Queries/GetLanguages.cs
+++ b/services/Directory/FilterLists.Directory.Application/Queries/GetLanguages.cs
@@ -50,6 +50,6 @@ public record LanguageVm
     {
         public string Iso6391 { get; init; } = null!;
         public string Name { get; init; } = null!;
-        public IEnumerable<int>? FilterListIds { get; init; }
+        public IReadOnlyCollection<int> FilterListIds { get; init; } = new HashSet<int>();
     }
 }
diff --git a/services/Directory/FilterLists.Directory.Application/Queries/GetLicenses.cs b/services/Directory/FilterLists.Directory.Application/Queries/GetLicenses.cs
index d60d4cd5d..23edc6cdb 100644
--- a/services/Directory/FilterLists.Directory.Application/Queries/GetLicenses.cs
+++ b/services/Directory/FilterLists.Directory.Application/Queries/GetLicenses.cs
@@ -52,6 +52,6 @@ public record LicenseVm
         public bool PermitsModification { get; init; }
         public bool PermitsDistribution { get; init; }
         public bool PermitsCommercialUse { get; init; }
-        public IEnumerable<int>? FilterListIds { get; init; }
+        public IReadOnlyCollection<int> FilterListIds { get; init; } = new HashSet<int>();
     }
 }
diff --git a/services/Directory/FilterLists.Directory.Application/Queries/GetMaintainers.cs b/services/Directory/FilterLists.Directory.Application/Queries/GetMaintainers.cs
index d3a9a2fd6..2aa4d47c2 100644
--- a/services/Directory/FilterLists.Directory.Application/Queries/GetMaintainers.cs
+++ b/services/Directory/FilterLists.Directory.Application/Queries/GetMaintainers.cs
@@ -52,6 +52,6 @@ public record MaintainerVm
         public Uri? Url { get; init; }
         public string? EmailAddress { get; init; }
         public string? TwitterHandle { get; init; }
-        public IEnumerable<int>? FilterListIds { get; init; }
+        public IReadOnlyCollection<int> FilterListIds { get; init; } = new HashSet<int>();
     }
 }
diff --git a/services/Directory/FilterLists.Directory.Application/Queries/GetSoftware.cs b/services/Directory/FilterLists.Directory.Application/Queries/GetSoftware.cs
index 10ad539f8..3a79a559f 100644
--- a/services/Directory/FilterLists.Directory.Application/Queries/GetSoftware.cs
+++ b/services/Directory/FilterLists.Directory.Application/Queries/GetSoftware.cs
@@ -52,6 +52,6 @@ public record SoftwareVm
         public Uri? HomeUrl { get; init; }
         public Uri? DownloadUrl { get; init; }
         public bool SupportsAbpUrlScheme { get; init; }
-        public IEnumerable<int>? SyntaxIds { get; init; }
+        public IReadOnlyCollection<int> SyntaxIds { get; init; } = new HashSet<int>();
     }
 }
diff --git a/services/Directory/FilterLists.Directory.Application/Queries/GetSyntaxes.cs b/services/Directory/FilterLists.Directory.Application/Queries/GetSyntaxes.cs
index dfdfc9ff1..773f58e0f 100644
--- a/services/Directory/FilterLists.Directory.Application/Queries/GetSyntaxes.cs
+++ b/services/Directory/FilterLists.Directory.Application/Queries/GetSyntaxes.cs
@@ -53,7 +53,7 @@ public record SyntaxVm
         public string Name { get; init; } = null!;
         public string? Description { get; init; }
         public Uri? Url { get; init; }
-        public IEnumerable<int>? FilterListIds { get; init; }
-        public IEnumerable<int>? SoftwareIds { get; init; }
+        public IReadOnlyCollection<int> FilterListIds { get; init; } = new HashSet<int>();
+        public IReadOnlyCollection<int> SoftwareIds { get; init; } = new HashSet<int>();
     }
 }
diff --git a/services/Directory/FilterLists.Directory.Application/Queries/GetTags.cs b/services/Directory/FilterLists.Directory.Application/Queries/GetTags.cs
index da3a45ac2..5e53674c5 100644
--- a/services/Directory/FilterLists.Directory.Application/Queries/GetTags.cs
+++ b/services/Directory/FilterLists.Directory.Application/Queries/GetTags.cs
@@ -49,6 +49,6 @@ public record TagVm
         public int Id { get; init; }
         public string Name { get; init; } = null!;
         public string? Description { get; init; }
-        public IEnumerable<int>? FilterListIds { get; init; }
+        public IReadOnlyCollection<int> FilterListIds { get; init; } = new HashSet<int>();
     }
 }