diff --git a/src/libraries/System.Formats.Cbor/src/System/Formats/Cbor/Reader/CborReader.Map.cs b/src/libraries/System.Formats.Cbor/src/System/Formats/Cbor/Reader/CborReader.Map.cs
index 1ca66a0537015a..e5dd5aced4bf1f 100644
--- a/src/libraries/System.Formats.Cbor/src/System/Formats/Cbor/Reader/CborReader.Map.cs
+++ b/src/libraries/System.Formats.Cbor/src/System/Formats/Cbor/Reader/CborReader.Map.cs
@@ -160,7 +160,18 @@ private void ValidateKeyUniqueness((int Offset, int Length) currentKeyEncodingRa
         {
             Debug.Assert(_currentKeyOffset != null);
 
-            HashSet<(int Offset, int Length)> keyEncodingRanges = GetKeyEncodingRanges();
+            HashSet<(int Offset, int Length)>? keyEncodingRanges = _keyEncodingRanges;
+            if (keyEncodingRanges is null)
+            {
+                if (_previousKeyEncodingRange is null)
+                {
+                    _previousKeyEncodingRange = currentKeyEncodingRange;
+                    return;
+                }
+
+                keyEncodingRanges = GetKeyEncodingRanges();
+                keyEncodingRanges.Add(_previousKeyEncodingRange.Value);
+            }
 
             if (!keyEncodingRanges.Add(currentKeyEncodingRange))
             {