diff --git a/.github/workflows/dotnet.yml b/.github/workflows/dotnet.yml
index b0eb411..3bf86a6 100644
--- a/.github/workflows/dotnet.yml
+++ b/.github/workflows/dotnet.yml
@@ -2,9 +2,9 @@ name: .NET
 
 on:
   push:
-    branches: [ master ]
+    branches: [ main ]
   pull_request:
-    branches: [ master ]
+    branches: [ main ]
 
 jobs:
   build:
diff --git a/Git.Test/TestObject.fs b/Git.Test/TestObject.fs
index e39dc92..7cbf4b0 100644
--- a/Git.Test/TestObject.fs
+++ b/Git.Test/TestObject.fs
@@ -79,3 +79,7 @@ module TestObject =
         for subStringEnd in 0 .. expected.Length - 1 do
             property expected.[0..subStringEnd]
             |> shouldEqual true
+
+            expected.[0..subStringEnd].ToUpperInvariant ()
+            |> property
+            |> shouldEqual true
diff --git a/Git/Object.fs b/Git/Object.fs
index 9e66dfa..58ff67d 100644
--- a/Git/Object.fs
+++ b/Git/Object.fs
@@ -21,17 +21,21 @@ type Object =
 module Object =
     /// Get the object hashes which match this start.
     let disambiguate (r : Repository) (startOfHash : string) : Hash list =
+        let objectDir = Repository.objectDir r
+
         match startOfHash.Length with
-        | 0 -> (Repository.objectDir r).EnumerateFiles ("*", SearchOption.AllDirectories)
+        | 0 -> objectDir.EnumerateFiles ("*", SearchOption.AllDirectories)
         | 1 ->
-            (Repository.objectDir r).EnumerateFiles ("*", SearchOption.AllDirectories)
-            |> Seq.filter (fun i ->
-                i.Directory.Name.Length > 0
-                && i.Directory.Name.[0] = startOfHash.[0]
-            )
+            if r.IsCaseSensitive then
+                objectDir.EnumerateDirectories ("*", SearchOption.AllDirectories)
+                |> Seq.filter (fun dir -> dir.Name.[0] = startOfHash.[0])
+                |> Seq.collect (fun dir -> dir.EnumerateFiles "*")
+            else
+                objectDir.EnumerateDirectories (sprintf "%c*" startOfHash.[0], SearchOption.AllDirectories)
+                |> Seq.collect (fun dir -> dir.EnumerateFiles "*")
         | 2 ->
             let subDir =
-                r.Fs.Path.Combine ((Repository.objectDir r).FullName, startOfHash)
+                r.Fs.Path.Combine (objectDir.FullName, startOfHash)
                 |> r.Fs.DirectoryInfo.FromDirectoryName
 
             if subDir.Exists then
@@ -40,15 +44,19 @@ module Object =
                 Seq.empty
         | _ ->
             let prefix = startOfHash.Substring (0, 2)
-            let suffix = startOfHash.Substring (2, startOfHash.Length - 2)
+            let suffix = startOfHash.Substring 2
 
             let subDir =
-                r.Fs.Path.Combine ((Repository.objectDir r).FullName, prefix)
+                r.Fs.Path.Combine (objectDir.FullName, prefix)
                 |> r.Fs.DirectoryInfo.FromDirectoryName
 
             if subDir.Exists then
-                subDir.EnumerateFiles ()
-                |> Seq.filter (fun i -> i.Name.StartsWith suffix)
+                if r.IsCaseSensitive then
+                    subDir.EnumerateFiles ()
+                    |> Seq.filter (fun i -> i.Name.StartsWith suffix)
+                else
+                    subDir.EnumerateFiles ()
+                    |> Seq.filter (fun i -> i.Name.StartsWith (suffix, true, null))
             else
                 Seq.empty
 
diff --git a/Git/Repository.fs b/Git/Repository.fs
index 9595ac5..3e1fb42 100644
--- a/Git/Repository.fs
+++ b/Git/Repository.fs
@@ -7,6 +7,7 @@ type Repository =
     private
         {
             Directory : IDirectoryInfo
+            IsCaseSensitive : bool
         }
 
     member this.Fs = this.Directory.FileSystem
@@ -38,34 +39,37 @@ module Repository =
         output
 
     let make (dir : IDirectoryInfo) : Repository option =
-        if
-            dir.Exists
-            && dir.EnumerateDirectories ()
-               |> Seq.map (fun i -> i.Name)
-               |> Seq.contains ".git"
-        then
-            Some { Directory = dir }
+        let fs = dir.FileSystem
+        let gitPath = fs.Path.Combine (dir.FullName, ".git")
+
+        if dir.Exists && fs.Directory.Exists gitPath then
+            {
+                Directory = dir
+                IsCaseSensitive =
+                    // Yes, if someone's made both `.git` and `.GIT` then we may think
+                    // the filesystem is case insensitive.
+                    not (fs.Directory.Exists (gitPath.ToUpperInvariant ()))
+            }
+            |> Some
         else
             None
 
     let init (dir : IDirectoryInfo) : Result<Repository, InitFailure> =
+        match make dir with
+        | Some _ -> Error AlreadyGit
+        | None ->
+
         if not dir.Exists then
             Error DirectoryDoesNotExist
-        elif
-            not
-            <| Seq.isEmpty (dir.EnumerateDirectories ".git")
-        then
-            Error AlreadyGit
         else
 
-        let r = { Directory = dir }
-
+        // TODO do this atomically
         let gitDir = createSubdir dir ".git"
         let objectDir = createSubdir gitDir "objects"
-        let packDir = createSubdir objectDir "pack"
-        let infoDir = createSubdir objectDir "info"
+        let _packDir = createSubdir objectDir "pack"
+        let _infoDir = createSubdir objectDir "info"
         let refsDir = createSubdir gitDir "refs"
-        let headsDir = createSubdir refsDir "heads"
-        let tagsDir = createSubdir refsDir "tags"
+        let _headsDir = createSubdir refsDir "heads"
+        let _tagsDir = createSubdir refsDir "tags"
 
-        r |> Ok
+        make dir |> Option.get |> Ok