diff --git a/TechnitiumLibrary.Net/Dns/DnsDatagram.cs b/TechnitiumLibrary.Net/Dns/DnsDatagram.cs
index 4f22d1b5..30fb659c 100644
--- a/TechnitiumLibrary.Net/Dns/DnsDatagram.cs
+++ b/TechnitiumLibrary.Net/Dns/DnsDatagram.cs
@@ -481,7 +481,7 @@ public static void SerializeDomainName(ReadOnlySpan<char> domain, Stream s, List
                     domain = domain.Slice(i + 1);
                 }
 
-                if (!Encoding.ASCII.TryGetBytes(label, labelBytes, out labelBytesLength))
+                if (!Encoding.Latin1.TryGetBytes(label, labelBytes, out labelBytesLength))
                     throw new DnsClientException("Invalid domain name: label cannot exceed 63 bytes.");
 
                 s.WriteByte((byte)labelBytesLength);
@@ -536,7 +536,7 @@ public static string DeserializeDomainName(Stream s, int maxDepth = 10, bool ign
                 Span<byte> label = buffer.Slice(0, labelLength);
                 s.ReadExactly(label);
 
-                if (!Encoding.ASCII.TryGetChars(label, domain.Slice(domainPosition), out _))
+                if (!Encoding.Latin1.TryGetChars(label, domain.Slice(domainPosition), out _))
                     throw new DnsClientException("Error while reading domain name: domain name length cannot exceed 255 bytes.");
 
                 domainPosition += labelLength;
@@ -566,6 +566,29 @@ public static string DeserializeDomainName(Stream s, int maxDepth = 10, bool ign
             return new string(domain.Slice(0, domainPosition));
         }
 
+        //RFC 4034 section 6.2 canonical form requires lowercasing ASCII A-Z only; every other byte (including
+        //the Latin-1 supplement range 0xC0-0xDE) must pass through unchanged. string.ToLowerInvariant() does
+        //full Unicode case folding and would corrupt those bytes, so domain names use this instead wherever
+        //the result feeds into a signed hash or a canonical-order comparison.
+        internal static string ToLowerInvariantAscii(string domain)
+        {
+            char[] buffer = null;
+
+            for (int i = 0; i < domain.Length; i++)
+            {
+                char c = domain[i];
+                if ((c >= 'A') && (c <= 'Z'))
+                {
+                    if (buffer is null)
+                        buffer = domain.ToCharArray();
+
+                    buffer[i] = (char)(c + 0x20);
+                }
+            }
+
+            return buffer is null ? domain : new string(buffer);
+        }
+
         public static int GetSerializeDomainNameLength(string domain)
         {
             if (domain.Length == 0)
diff --git a/TechnitiumLibrary.Net/Dns/ResourceRecords/CanonicallySerializedResourceRecord.cs b/TechnitiumLibrary.Net/Dns/ResourceRecords/CanonicallySerializedResourceRecord.cs
index 28d6f368..06866d68 100644
--- a/TechnitiumLibrary.Net/Dns/ResourceRecords/CanonicallySerializedResourceRecord.cs
+++ b/TechnitiumLibrary.Net/Dns/ResourceRecords/CanonicallySerializedResourceRecord.cs
@@ -56,7 +56,7 @@ public static CanonicallySerializedResourceRecord Create(string name, DnsResourc
 
             //serialize owner name | type | class | Original TTL | RDATA length
             buffer.SetLength(0);
-            DnsDatagram.SerializeDomainName(name.ToLowerInvariant(), buffer);
+            DnsDatagram.SerializeDomainName(DnsDatagram.ToLowerInvariantAscii(name), buffer);
             DnsDatagram.WriteUInt16NetworkOrder((ushort)type, buffer);
             DnsDatagram.WriteUInt16NetworkOrder((ushort)@class, buffer);
             DnsDatagram.WriteUInt32NetworkOrder(originalTtl, buffer);
diff --git a/TechnitiumLibrary.Net/Dns/ResourceRecords/DnsNSECRecordData.cs b/TechnitiumLibrary.Net/Dns/ResourceRecords/DnsNSECRecordData.cs
index 04111ab3..da6a102c 100644
--- a/TechnitiumLibrary.Net/Dns/ResourceRecords/DnsNSECRecordData.cs
+++ b/TechnitiumLibrary.Net/Dns/ResourceRecords/DnsNSECRecordData.cs
@@ -107,8 +107,8 @@ public static bool IsDomainCovered(string ownerName, string nextDomainName, stri
 
         public static int CanonicalComparison(string domain1, string domain2)
         {
-            string[] labels1 = domain1.ToLowerInvariant().Split('.');
-            string[] labels2 = domain2.ToLowerInvariant().Split('.');
+            string[] labels1 = DnsDatagram.ToLowerInvariantAscii(domain1).Split('.');
+            string[] labels2 = DnsDatagram.ToLowerInvariantAscii(domain2).Split('.');
 
             int minLength = labels1.Length;
 
@@ -117,7 +117,7 @@ public static int CanonicalComparison(string domain1, string domain2)
 
             for (int i = 0; i < minLength; i++)
             {
-                int value = CanonicalComparison(Encoding.ASCII.GetBytes(labels1[labels1.Length - 1 - i]), Encoding.ASCII.GetBytes(labels2[labels2.Length - 1 - i]));
+                int value = CanonicalComparison(Encoding.Latin1.GetBytes(labels1[labels1.Length - 1 - i]), Encoding.Latin1.GetBytes(labels2[labels2.Length - 1 - i]));
                 if (value != 0)
                     return value;
             }