--- a/content/base/src/nsDocumentEncoder.cpp
+++ b/content/base/src/nsDocumentEncoder.cpp
@@ -554,16 +554,22 @@ ConvertAndWrite(const nsAString& aString
   const char16_t* unicodeBuf = flat.get();
   int32_t unicodeLength = aString.Length();
   int32_t startLength = unicodeLength;
 
   rv = aEncoder->GetMaxLength(unicodeBuf, unicodeLength, &charLength);
   startCharLength = charLength;
   NS_ENSURE_SUCCESS(rv, rv);
 
+  if (!charLength) {
+    // Nothing to write.  Besides, a length 0 string has an immutable buffer, so
+    // attempts to null-terminate it will crash.
+    return NS_OK;
+  }
+
   nsAutoCString charXferString;
   if (!charXferString.SetLength(charLength, fallible_t()))
     return NS_ERROR_OUT_OF_MEMORY;
 
   char* charXferBuf = charXferString.BeginWriting();
   nsresult convert_rv = NS_OK;
 
   do {