diff --git a/src/libraries/System.Memory/tests/Base64/Base64DecoderUnitTests.cs b/src/libraries/System.Memory/tests/Base64/Base64DecoderUnitTests.cs
index 4e11de38490591..0d792b26682d60 100644
--- a/src/libraries/System.Memory/tests/Base64/Base64DecoderUnitTests.cs
+++ b/src/libraries/System.Memory/tests/Base64/Base64DecoderUnitTests.cs
@@ -57,6 +57,19 @@ public void BasicDecodingInvalidInputLength()
             }
         }
 
+        [Fact]
+        public void BasicDecodingInvalidInputWithSlicedSource()
+        {
+            ReadOnlySpan<byte> source = stackalloc byte[] { (byte)'A', (byte)'B', (byte)'C', (byte)'D' };
+            Span<byte> decodedBytes = stackalloc byte[128];
+
+            source = source[..3];   // now it's invalid as only 3 bytes are present
+
+            Assert.Equal(OperationStatus.InvalidData, Base64.DecodeFromUtf8(source, decodedBytes, out int consumed, out int decodedByteCount));
+            Assert.Equal(0, consumed);
+            Assert.Equal(0, decodedByteCount);
+        }
+
         [Fact]
         public void BasicDecodingWithFinalBlockFalse()
         {
diff --git a/src/libraries/System.Private.CoreLib/src/System/Buffers/Text/Base64Decoder.cs b/src/libraries/System.Private.CoreLib/src/System/Buffers/Text/Base64Decoder.cs
index 2ec9959536427e..f4ebc942ac331a 100644
--- a/src/libraries/System.Private.CoreLib/src/System/Buffers/Text/Base64Decoder.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Buffers/Text/Base64Decoder.cs
@@ -101,7 +101,7 @@ public static unsafe OperationStatus DecodeFromUtf8(ReadOnlySpan<byte> utf8, Spa
                 }
 
                 ref sbyte decodingMap = ref MemoryMarshal.GetReference(DecodingMap);
-                srcMax = srcBytes + (uint)maxSrcLength;
+                srcMax = srcBytes + maxSrcLength;
 
                 while (src < srcMax)
                 {