diff --git a/myst_parser/parse_directives.py b/myst_parser/parse_directives.py
index c3c4375d..352f67bf 100644
--- a/myst_parser/parse_directives.py
+++ b/myst_parser/parse_directives.py
@@ -147,7 +147,7 @@ def parse_directive_options(
             raise DirectiveParsingError(f"Unknown option: {name}")
         if not isinstance(value, str):
             if value is True or value is None:
-                value = ""  # flag converter requires no argument
+                value = None  # flag converter requires no argument
             elif isinstance(value, (int, float, datetime.date, datetime.datetime)):
                 # convertor always requires string input
                 value = str(value)
diff --git a/tests/test_renderers/fixtures/sphinx_directives.md b/tests/test_renderers/fixtures/sphinx_directives.md
index 3b737c86..a84040ef 100644
--- a/tests/test_renderers/fixtures/sphinx_directives.md
+++ b/tests/test_renderers/fixtures/sphinx_directives.md
@@ -45,8 +45,9 @@ highlight (`sphinx.directives.code.Highlight`):
 code-block (`sphinx.directives.code.CodeBlock`):
 .
 ```{code-block}
+:dedent:
 
-a=1
+    a=1
 ```
 .
 <document source="notset">