diff --git a/csharp/src/Google.Protobuf/WritingPrimitives.cs b/csharp/src/Google.Protobuf/WritingPrimitives.cs
index 823cdb7f49db..70f96e965de8 100644
--- a/csharp/src/Google.Protobuf/WritingPrimitives.cs
+++ b/csharp/src/Google.Protobuf/WritingPrimitives.cs
@@ -182,7 +182,8 @@ public static void WriteString(ref Span<byte> buffer, ref WriterInternalState st
                     // If 64bit, and value has at least 4 chars, process 4 chars at a time.
                     if (IntPtr.Size == 8 && value.Length >= 4)
                     {
-                        ref byte sourceBytes = ref Unsafe.As<char, byte>(ref MemoryMarshal.GetReference(value.AsSpan()));
+                        ref char sourceChars = ref MemoryMarshal.GetReference(value.AsSpan());
+                        ref byte sourceBytes = ref Unsafe.As<char, byte>(ref sourceChars);
                         ref byte destinationBytes = ref MemoryMarshal.GetReference(buffer.Slice(state.position));
 
                         // Process 4 chars at a time until there are less than 4 remaining.
@@ -201,7 +202,7 @@ public static void WriteString(ref Span<byte> buffer, ref WriterInternalState st
                         // so use them to perform array operations without bounds checks.
                         for (; currentIndex < length; currentIndex++)
                         {
-                            Unsafe.AddByteOffset(ref destinationBytes, (IntPtr)currentIndex) = Unsafe.AddByteOffset(ref sourceBytes, (IntPtr)(currentIndex * 2));
+                            Unsafe.AddByteOffset(ref destinationBytes, (IntPtr)currentIndex) = (byte)Unsafe.AddByteOffset(ref sourceChars, (IntPtr)(currentIndex * 2));
                         }
                     }
                     else