diff --git a/proto/caosdb/entity/v1alpha1/main.proto b/proto/caosdb/entity/v1alpha1/main.proto
index 9c11bb3973672d27fc1504a6c10cfb2b3f77ef83..f920154ec81f126bdff8407c93741b78a25051e5 100644
--- a/proto/caosdb/entity/v1alpha1/main.proto
+++ b/proto/caosdb/entity/v1alpha1/main.proto
@@ -24,12 +24,25 @@ option  java_multiple_files = true;
 option  java_package        = "org.caosdb.api.entity.v1alpha1";
 package caosdb.entity.v1alpha1;
 
+// Entity messages codes.
+enum MessageCode {
+  // Unspecific code. Use this for anything where a client is not expected to
+  // handle this message in a specific way.
+  MESSAGE_CODE_UNSPECIFIED = 0;
+  // Unknown code. This value is a fall-back which may be used when the message
+  // code was not in this list. An unknown error code is a sign of a wrong otr
+  // out-dated implementation on either side.
+  MESSAGE_CODE_UNKNOWN = 1;
+  // The requested entity (retrieve, delete, or update) does not exist.
+  MESSAGE_CODE_ENTITY_DOES_NOT_EXIST = 2;
+}
+
 // Messages are used by server's or client's to transport *transient*
 // information, e.g. general information about a transaction, warnings, or
 // errors.
 message Message {
   // The code is a machine-readable identifier.
-  uint32 code = 1;
+  int32 code = 1;
   // A short description of the message which is indented for human readers.
   string description = 2;
 }