diff --git a/mesonbuild/coredata.py b/mesonbuild/coredata.py
index e99f99e17eba..5206d9625d5c 100644
--- a/mesonbuild/coredata.py
+++ b/mesonbuild/coredata.py
@@ -289,7 +289,7 @@ def __init__(self, cmd_options: SharedCMDOptions, scratch_dir: str, meson_comman
         # Only to print a warning if it changes between Meson invocations.
         self.config_files = self.__load_config_files(cmd_options, scratch_dir, 'native')
         self.builtin_options_libdir_cross_fixup()
-        self.init_builtins('')
+        self.init_builtins()
 
     @staticmethod
     def __load_config_files(cmd_options: SharedCMDOptions, scratch_dir: str, ftype: str) -> T.List[str]:
@@ -356,7 +356,7 @@ def builtin_options_libdir_cross_fixup(self) -> None:
         if self.cross_files:
             options.BUILTIN_OPTIONS[OptionKey('libdir')].default = 'lib'
 
-    def init_builtins(self, subproject: str) -> None:
+    def init_builtins(self) -> None:
         # Create builtin options with default values
         for key, opt in options.BUILTIN_OPTIONS.items():
             self.add_builtin_option(self.optstore, key, opt)
@@ -448,9 +448,7 @@ def set_option(self, key: OptionKey, value, first_invocation: bool = False) -> b
         except KeyError:
             raise MesonException(f'Tried to set unknown builtin option {str(key)}')
 
-        changed = self.optstore.set_value(key.name, key.subproject, value)
-        if changed and opt.readonly and not first_invocation:
-            raise MesonException(f'Tried modify read only option {str(key)!r}')
+        changed = self.optstore.set_value(key, value)
         dirty |= changed
 
         if key.name == 'buildtype':
@@ -692,6 +690,8 @@ def set_default_options(self, default_options: T.MutableMapping[OptionKey, str],
         options = OrderedDict()
 
         for k, v in env.options.items():
+            if isinstance(k, str):
+                k = OptionKey.from_string(k)
             # If this is a subproject, don't use other subproject options
             if k.subproject and k.subproject != subproject:
                 continue
diff --git a/mesonbuild/options.py b/mesonbuild/options.py
index d8c351e5b8b3..a0a216be2a75 100644
--- a/mesonbuild/options.py
+++ b/mesonbuild/options.py
@@ -829,7 +829,7 @@ def sanitize_dir_option_value(self, prefix: str, option: OptionKey, value: T.Any
         except TypeError:
             return value
         if option.name.endswith('dir') and value.is_absolute() and \
-           option not in options.BUILTIN_DIR_NOPREFIX_OPTIONS:
+           option not in BUILTIN_DIR_NOPREFIX_OPTIONS:
             try:
                 # Try to relativize the path.
                 value = value.relative_to(prefix)
@@ -856,7 +856,11 @@ def set_value(self, key: T.Union[OptionKey, str], new_value: 'T.Any') -> bool:
                 new_value = self.sanitize_dir_option_value(prefix, key, new_value)
         if key not in self.options:
            raise MesonException(f'Internal error, tried to access non-existing option {key.name}.')
-        return self.options[key].set_value(new_value)
+        valobj = self.options[key]
+        changed = valobj.set_value(new_value)
+        if valobj.readonly and changed:
+            raise MesonException(f'Tried modify read only option {str(key)!r}')
+        return changed
 
     def set_option(self, name: str, subproject: T.Optional[str], new_value: str):
         key = OptionKey(name, subproject)
@@ -886,7 +890,13 @@ def replace(v):
             dirty |= opt.set_value(new_value)
             return dirty
 
-        return opt.set_value(new_value)
+        changed = opt.set_value(new_value)
+
+        if opt.readonly and changed:
+            raise MesonException(f'Tried modify read only option {str(key)!r}')
+
+
+        return changed
 
     # FIXME, this should be removed.or renamed to "change_type_of_existing_object" or something like that
     def set_value_object(self, key: T.Union[OptionKey, str], new_object: 'UserOption[T.Any]') -> bool:
@@ -1094,7 +1104,10 @@ def set_from_top_level_project_call(self, project_default_options, cmd_line_opti
                 else:
                     self.pending_project_options[key] = valstr
         for keystr, valstr in cmd_line_options.items():
-            key = OptionKey.from_string(keystr)
+            if isinstance(keystr, str):
+                key = OptionKey.from_string(keystr)
+            else:
+                key = keystr
             if key.subproject is None:
                 projectkey = key.evolve(subproject='')
                 if key in self.options:
diff --git a/unittests/platformagnostictests.py b/unittests/platformagnostictests.py
index b97ebfc9895f..4ca289881c24 100644
--- a/unittests/platformagnostictests.py
+++ b/unittests/platformagnostictests.py
@@ -174,14 +174,14 @@ def test_change_backend(self):
             self.setconf('-Dbackend=none')
         self.assertIn("ERROR: Tried modify read only option 'backend'", cm.exception.stdout)
 
-        # Reconfigure with a different backend is not allowed
-        with self.assertRaises(subprocess.CalledProcessError) as cm:
-            self.init(testdir, extra_args=['--reconfigure', '--backend=none'])
-        self.assertIn("ERROR: Tried modify read only option 'backend'", cm.exception.stdout)
+        # Check that the new value was not written in the store.
+        self.assertEqual(self.getconf('backend'), 'ninja')
 
         # Wipe with a different backend is allowed
         self.init(testdir, extra_args=['--wipe', '--backend=none'])
 
+        self.assertEqual(self.getconf('backend'), 'none')
+
     def test_validate_dirs(self):
         testdir = os.path.join(self.common_test_dir, '1 trivial')