diff --git a/Adobe-error-codes.md b/Adobe-error-codes.md
index af31899..7de4a77 100644
--- a/Adobe-error-codes.md
+++ b/Adobe-error-codes.md
@@ -295,6 +295,12 @@ The solution to this is to create a new AdobeID and then immediately link the an
 
 This error is similar to [E_ADEPT_NO_ANONYMOUS_USER](#E_ADEPT_NO_ANONYMOUS_USER), with the difference being that with `E_AUTH_USER_ALREADY_REGISTERED` the client already knows the authorization change is not going to work, while with `E_ADEPT_NO_ANONYMOUS_USER` it's the server that denies the link request. 
 
+### E_AUTH_USER_AUTH
+
+This error does not appear in any Adobe documentation, but it seems to occur if the "adept:signature" parameter of an XML request is either wrong or can't be parsed. Either there's a server issue or the program you're using to connect to the Adobe servers is buggy. 
+
+If this happens with the Calibre plugin, please open a bug report.
+
 ### E_AUTH_USERID_INUSE
 
 This error is also related to linking anonymous authorizations to an AdobeID. It occurs if the unique user UUID of your anonymous authorization has already been linked to an AdobeID. For example, if you have an anonymous authorization in the plugin, then export that to another computer so you have an identical clone of the authorization. If you then try to link that authorization to two different AdobeIDs on the two computers, you will get the error `E_AUTH_USERID_INUSE` as the anonymous authorization has already been converted to an AdobeID. In normal operation, this should never occur.