diff --git a/dlls/rpcrt4/rpc_defs.h b/dlls/rpcrt4/rpc_defs.h
index 8514967a9c9..434d39699ee 100644
--- a/dlls/rpcrt4/rpc_defs.h
+++ b/dlls/rpcrt4/rpc_defs.h
@@ -139,6 +139,13 @@ typedef struct
   unsigned short num_data_items;
 } RpcPktHttpHdr;
 
+/* AUTH3 packet */
+typedef struct
+{
+  RpcPktCommonHdr common;
+  unsigned int pad; /* ignored */
+} RpcPktAuth3Hdr;
+
 /* Union representing all possible packet headers */
 typedef union
 {
@@ -150,6 +157,7 @@ typedef union
   RpcPktBindAckHdr bind_ack;
   RpcPktBindNAckHdr bind_nack;
   RpcPktHttpHdr http;
+  RpcPktAuth3Hdr auth3;
 } RpcPktHdr;
 
 typedef struct
diff --git a/dlls/rpcrt4/rpc_message.c b/dlls/rpcrt4/rpc_message.c
index 45a0398c846..0144f85dbfd 100644
--- a/dlls/rpcrt4/rpc_message.c
+++ b/dlls/rpcrt4/rpc_message.c
@@ -59,7 +59,7 @@ DWORD RPCRT4_GetHeaderSize(const RpcPktHdr *Header)
     sizeof(Header->request), 0, sizeof(Header->response),
     sizeof(Header->fault), 0, 0, 0, 0, 0, 0, 0, sizeof(Header->bind),
     sizeof(Header->bind_ack), sizeof(Header->bind_nack),
-    0, 0, sizeof(Header->common), 0, 0, 0, sizeof(Header->http)
+    0, 0, sizeof(Header->auth3), 0, 0, 0, sizeof(Header->http)
   };
   ULONG ret = 0;
   
@@ -217,12 +217,12 @@ static RpcPktHdr *RPCRT4_BuildAuthHeader(ULONG DataRepresentation)
   RpcPktHdr *header;
 
   header = HeapAlloc(GetProcessHeap(), HEAP_ZERO_MEMORY,
-                     sizeof(header->common));
+                     sizeof(header->auth3));
   if (header == NULL)
     return NULL;
 
   RPCRT4_BuildCommonHeader(header, PKT_AUTH3, DataRepresentation);
-  header->common.frag_len = sizeof(header->common);
+  header->common.frag_len = sizeof(header->auth3);
 
   return header;
 }