diff --git a/src/RestSharp/Request/RequestContent.cs b/src/RestSharp/Request/RequestContent.cs
index 9df44d6a9..fd6adaea1 100644
--- a/src/RestSharp/Request/RequestContent.cs
+++ b/src/RestSharp/Request/RequestContent.cs
@@ -27,9 +27,17 @@ class RequestContent : IDisposable {
     readonly List<Stream> _streams = new();
 
     internal static readonly string[] ContentHeaders = {
-        Allow, Expires,
-        ContentDisposition, ContentEncoding, ContentLanguage, ContentLength, ContentLocation, ContentRange, ContentType, ContentMD5,
-        LastModified
+        Allow.ToLower(),
+        Expires.ToLower(),
+        ContentDisposition.ToLower(),
+        ContentEncoding.ToLower(),
+        ContentLanguage.ToLower(),
+        ContentLength.ToLower(),
+        ContentLocation.ToLower(),
+        ContentRange.ToLower(),
+        ContentType.ToLower(),
+        ContentMD5.ToLower(),
+        LastModified.ToLower(),
     };
 
     HttpContent? Content { get; set; }
@@ -172,7 +180,7 @@ void AddPostParameters(ParametersCollection? postParameters) {
 
     void AddHeaders() {
         var contentHeaders = _request.Parameters
-            .Where(x => x.Type == ParameterType.HttpHeader && ContentHeaders.Contains(x.Name))
+            .Where(x => x.Type == ParameterType.HttpHeader && ContentHeaders.Contains(x.Name?.ToLower()))
             .ToArray();
 
         if (contentHeaders.Length > 0 && Content == null) {
@@ -208,4 +216,4 @@ public void Dispose() {
         _streams.ForEach(x => x.Dispose());
         Content?.Dispose();
     }
-}
\ No newline at end of file
+}
diff --git a/test/RestSharp.Tests/RestContentTests.cs b/test/RestSharp.Tests/RestContentTests.cs
new file mode 100644
index 000000000..17f80df8d
--- /dev/null
+++ b/test/RestSharp.Tests/RestContentTests.cs
@@ -0,0 +1,18 @@
+using System.Net;
+
+namespace RestSharp.Tests;
+
+public class RestContentTests {
+    [Fact]
+    public void RestContent_CaseInsensitiveHeaders() {
+        var myContentType = "application/x-custom";
+        var request = new RestRequest("resource");
+        request.AddHeader("coNteNt-TypE", myContentType);
+        var client = new RestClient();
+        var content = new RequestContent(client, request);
+
+        var httpContent = content.BuildContent();
+
+        Assert.Equal(myContentType, httpContent.Headers.ContentType.MediaType);
+    }
+}