diff --git a/mesonbuild/ast/interpreter.py b/mesonbuild/ast/interpreter.py
index 783387df1e29..0887fa2ebab1 100644
--- a/mesonbuild/ast/interpreter.py
+++ b/mesonbuild/ast/interpreter.py
@@ -49,7 +49,7 @@
 if T.TYPE_CHECKING:
     from .visitor import AstVisitor
     from ..interpreter import Interpreter
-    from ..interpreterbase import TYPE_nkwargs, TYPE_var
+    from ..interpreterbase import SubProject, TYPE_nkwargs, TYPE_var
     from ..mparser import (
         AndNode,
         ComparisonNode,
@@ -84,8 +84,9 @@ class MockRunTarget(MesonInterpreterObject):
 _T = T.TypeVar('_T')
 _V = T.TypeVar('_V')
 
+
 class AstInterpreter(InterpreterBase):
-    def __init__(self, source_root: str, subdir: str, subproject: str, visitors: T.Optional[T.List[AstVisitor]] = None):
+    def __init__(self, source_root: str, subdir: str, subproject: SubProject, visitors: T.Optional[T.List[AstVisitor]] = None):
         super().__init__(source_root, subdir, subproject)
         self.visitors = visitors if visitors is not None else []
         self.processed_buildfiles: T.Set[str] = set()
diff --git a/mesonbuild/ast/introspection.py b/mesonbuild/ast/introspection.py
index 49e58f43d101..095f30c14fd1 100644
--- a/mesonbuild/ast/introspection.py
+++ b/mesonbuild/ast/introspection.py
@@ -14,7 +14,7 @@
 from .. import coredata as cdata
 from ..build import Executable, Jar, SharedLibrary, SharedModule, StaticLibrary
 from ..compilers import detect_compiler_for
-from ..interpreterbase import InvalidArguments
+from ..interpreterbase import InvalidArguments, SubProject
 from ..mesonlib import MachineChoice, OptionKey
 from ..mparser import BaseNode, ArithmeticNode, ArrayNode, ElementaryNode, IdNode, FunctionNode, BaseStringNode
 from .interpreter import AstInterpreter
@@ -51,7 +51,7 @@ def __init__(self,
                  backend: str,
                  visitors: T.Optional[T.List[AstVisitor]] = None,
                  cross_file: T.Optional[str] = None,
-                 subproject: str = '',
+                 subproject: SubProject = SubProject(''),
                  subproject_dir: str = 'subprojects',
                  env: T.Optional[environment.Environment] = None):
         visitors = visitors if visitors is not None else []
@@ -126,7 +126,7 @@ def func_project(self, node: BaseNode, args: T.List[TYPE_var], kwargs: T.Dict[st
             if os.path.isdir(subprojects_dir):
                 for i in os.listdir(subprojects_dir):
                     if os.path.isdir(os.path.join(subprojects_dir, i)):
-                        self.do_subproject(i)
+                        self.do_subproject(SubProject(i))
 
         self.coredata.init_backend_options(self.backend)
         options = {k: v for k, v in self.environment.options.items() if k.is_backend()}
@@ -135,7 +135,7 @@ def func_project(self, node: BaseNode, args: T.List[TYPE_var], kwargs: T.Dict[st
         self._add_languages(proj_langs, True, MachineChoice.HOST)
         self._add_languages(proj_langs, True, MachineChoice.BUILD)
 
-    def do_subproject(self, dirname: str) -> None:
+    def do_subproject(self, dirname: SubProject) -> None:
         subproject_dir_abs = os.path.join(self.environment.get_source_dir(), self.subproject_dir)
         subpr = os.path.join(subproject_dir_abs, dirname)
         try: