diff --git a/sphinx_d2lang/d2lang.py b/sphinx_d2lang/d2lang.py
index e7e65d5..73c7359 100644
--- a/sphinx_d2lang/d2lang.py
+++ b/sphinx_d2lang/d2lang.py
@@ -17,7 +17,7 @@ class D2langDirective(Directive):
         diag_source = self.content
         with tempfile.NamedTemporaryFile() as fp:
             for line in self.content:
-                fp.write(line)
+                fp.write(bytes(line,'utf-8'))
             cmd_line = '%s %s %s' % (d2_bin, fp.name, srcdir + "/test.svg")
         args = shlex.split(cmd_line)
         subprocess.run(args)