diff --git a/dlls/rpcrt4/rpc_transport.c b/dlls/rpcrt4/rpc_transport.c
index e8bdd49266d784b8f1b77aa759536fea89cece35..c073a95774326fa268ec6ed9b2e8d342d680ed24 100644
--- a/dlls/rpcrt4/rpc_transport.c
+++ b/dlls/rpcrt4/rpc_transport.c
@@ -2383,7 +2383,7 @@ static RPC_STATUS rpcrt4_http_prepare_out_pipe(HINTERNET out_request, RpcHttpAsy
 
 static UINT encode_base64(const char *bin, unsigned int len, WCHAR *base64)
 {
-    static char enc[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
+    static const char enc[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
     UINT i = 0, x;
 
     while (len > 0)