diff --git a/test/eiproto.py.tmpl b/test/eiproto.py.tmpl
index 9769e07..33fbbbf 100644
--- a/test/eiproto.py.tmpl
+++ b/test/eiproto.py.tmpl
@@ -174,7 +174,7 @@ class Interface:
             elif sig in ["s"]:
                 encoding.append("I")
                 arguments.append(len(arg) + 1)
-                slen = ((len(arg) + 3) // 4) * 4
+                slen = ((len(arg) + 1 + 3) // 4) * 4
                 encoding.append(f"{slen}s")
                 arg = arg.encode("utf8")
             elif sig in ["h"]: