diff --git a/src/dvsim/flow/one_shot.py b/src/dvsim/flow/one_shot.py
index c717eea2..71ffdb37 100644
--- a/src/dvsim/flow/one_shot.py
+++ b/src/dvsim/flow/one_shot.py
@@ -53,7 +53,7 @@ def __init__(self, flow_cfg_file, hjson_data, args, mk_config) -> None:
         self.dut = ""
         self.fusesoc_core = ""
         self.ral_spec = ""
-        self.build_modes = []
+        self.build_modes: Sequence[BuildMode] = []
         self.run_modes = []
         self.regressions = []
         self.max_msg_count = -1
@@ -110,17 +110,34 @@ def _expand(self) -> None:
     def _purge(self) -> None:
         assert self.scratch_path
         log.info("Purging scratch path %s", self.scratch_path)
-        rm_path(self.scratch_path)
+        rm_path(Path(self.scratch_path))
 
     def _create_objects(self) -> None:
-        # Create build and run modes objects
-        build_modes = Mode.create_modes(BuildMode, self.build_modes)
-        self.build_modes = build_modes
-
-        # All defined build modes are being built, h
-        # ence extend all with the global opts.
-        for build_mode in build_modes:
-            build_mode.build_opts.extend(self.build_opts)
+        # Create build modes objects based on the names that were stored in
+        # self.build_modes by the superclass constructor's call to
+        # _merge_hjson.
+        #
+        # We've lied to the type checker about the type of self.build_modes,
+        # giving it the type that it will contain after this function is
+        # complete.
+        mode_dicts: list[dict[object, object]] = []
+        for mode_dict in self.build_modes:
+            if not isinstance(mode_dict, dict):
+                msg = f"Found a build mode item of {mode_dict} when we expected a dict."
+                raise TypeError(msg)
+
+            mode_dicts.append(mode_dict)
+
+        base_modes: Sequence[Mode] = Mode.create_modes(BuildMode, mode_dicts)
+
+        # All defined build modes are being built, so should be extended with
+        # the global opts. Tighten up the inferred type at the same time.
+        self.build_modes = []
+        for mode in base_modes:
+            if not isinstance(mode, BuildMode):
+                raise TypeError("create_modes returned the wrong type")
+            mode.build_opts.extend(self.build_opts)
+            self.build_modes.append(mode)
 
     def _print_list(self) -> None:
         for list_item in self.list_items:
diff --git a/src/dvsim/modes.py b/src/dvsim/modes.py
index 9f944518..f7ae265c 100644
--- a/src/dvsim/modes.py
+++ b/src/dvsim/modes.py
@@ -146,7 +146,7 @@ def merge_mode(self, mode: "Mode") -> None:
         return True
 
     @staticmethod
-    def create_modes(mode_type: "type[Mode]", mdicts: Mapping) -> Sequence["Mode"]:
+    def create_modes(mode_type: "type[Mode]", mdicts: Sequence[Mapping]) -> Sequence["Mode"]:
         """Create modes of type mode_type.
 
         Use the given list of raw dicts Process dependencies.