more specific check for object/string combo

Author: jorisvandenbossche

pandas/core/indexes/base.py

@@ -6556,10 +6556,16 @@ def _maybe_cast_listlike_indexer(self, target) -> Index:
         """
         Analogue to maybe_cast_indexer for get_indexer instead of get_loc.
         """
-        if not hasattr(target, "dtype") and self.dtype == object:
-            # Avoid inference for object since we are casting back later anyway
-            return Index(target, dtype=self.dtype)
-        return ensure_index(target)
+        target_index = ensure_index(target)
+        if (
+            not hasattr(target, "dtype")
+            and self.dtype == object
+            and target_index.dtype == "string"
+        ):
+            # If we started with a list-like, avoid inference to string dtype if self
+            # is object dtype (coercing to string dtype will alter the missing values)
+            target_index = Index(target, dtype=self.dtype)
+        return target_index
 
     @final
     def _validate_indexer(