diff --git a/src/System.Private.CoreLib/shared/System/IO/StreamReader.cs b/src/System.Private.CoreLib/shared/System/IO/StreamReader.cs
index 9d836c0b98b2..61accbf8996e 100644
--- a/src/System.Private.CoreLib/shared/System/IO/StreamReader.cs
+++ b/src/System.Private.CoreLib/shared/System/IO/StreamReader.cs
@@ -135,17 +135,25 @@ public StreamReader(Stream stream, Encoding encoding, bool detectEncodingFromByt
         {
         }
 
-        public StreamReader(Stream stream, Encoding encoding, bool detectEncodingFromByteOrderMarks, int bufferSize, bool leaveOpen)
+        public StreamReader(Stream stream, Encoding? encoding = null, bool detectEncodingFromByteOrderMarks = true, int bufferSize = -1, bool leaveOpen = false)
         {
-            if (stream == null || encoding == null)
+            if (stream == null)
             {
-                throw new ArgumentNullException(stream == null ? nameof(stream) : nameof(encoding));
+                throw new ArgumentNullException(nameof(stream));
+            }
+            if (encoding == null)
+            {
+                encoding = Encoding.UTF8;
             }
             if (!stream.CanRead)
             {
                 throw new ArgumentException(SR.Argument_StreamNotReadable);
             }
-            if (bufferSize <= 0)
+            if (bufferSize == -1)
+            {
+                bufferSize = DefaultBufferSize;
+            }
+            else if (bufferSize <= 0)
             {
                 throw new ArgumentOutOfRangeException(nameof(bufferSize), SR.ArgumentOutOfRange_NeedPosNum);
             }
diff --git a/src/System.Private.CoreLib/shared/System/IO/StreamWriter.cs b/src/System.Private.CoreLib/shared/System/IO/StreamWriter.cs
index 3a999195bc1d..856dd27280de 100644
--- a/src/System.Private.CoreLib/shared/System/IO/StreamWriter.cs
+++ b/src/System.Private.CoreLib/shared/System/IO/StreamWriter.cs
@@ -90,18 +90,26 @@ public StreamWriter(Stream stream, Encoding encoding, int bufferSize)
         {
         }
 
-        public StreamWriter(Stream stream, Encoding encoding, int bufferSize, bool leaveOpen)
+        public StreamWriter(Stream stream, Encoding? encoding = null, int bufferSize = -1, bool leaveOpen = false)
             : base(null) // Ask for CurrentCulture all the time
         {
-            if (stream == null || encoding == null)
+            if (stream == null)
             {
-                throw new ArgumentNullException(stream == null ? nameof(stream) : nameof(encoding));
+                throw new ArgumentNullException(nameof(stream));
+            }
+            if (encoding == null)
+            {
+                encoding = UTF8NoBOM;
             }
             if (!stream.CanWrite)
             {
                 throw new ArgumentException(SR.Argument_StreamNotWritable);
             }
-            if (bufferSize <= 0)
+            if (bufferSize == -1)
+            {
+                bufferSize = DefaultBufferSize;
+            }
+            else if (bufferSize <= 0)
             {
                 throw new ArgumentOutOfRangeException(nameof(bufferSize), SR.ArgumentOutOfRange_NeedPosNum);
             }
diff --git a/tests/CoreFX/CoreFX.issues.json b/tests/CoreFX/CoreFX.issues.json
index e8428f5a928b..5fb301498fa5 100644
--- a/tests/CoreFX/CoreFX.issues.json
+++ b/tests/CoreFX/CoreFX.issues.json
@@ -775,6 +775,10 @@
                 {
                     "name": "System.IO.Tests.StreamWriterWithBufferedStream_CloseTests.AfterCloseThrows",
                     "reason": "Readers/writers changed to not null out base stream on dispose"
+                },
+                {
+                    "name": "System.IO.Tests.CtorTests.NullEncodingThrows",
+                    "reason": "StreamReader/StreamWriter changed to accept null encoding"
                 }
             ]
         }