diff --git a/xarray/core/dtypes.py b/xarray/core/dtypes.py
index c959a7f2536..a9ee01485e1 100644
--- a/xarray/core/dtypes.py
+++ b/xarray/core/dtypes.py
@@ -67,7 +67,7 @@ def maybe_promote(dtype: np.dtype) -> tuple[np.dtype, Any]:
         # for now, we always promote string dtypes to object for consistency with existing behavior
         # TODO: refactor this once we have a better way to handle numpy vlen-string dtypes
         dtype_ = object
-        fill_value = np.nan
+        fill_value = ""
     elif isdtype(dtype, "real floating"):
         dtype_ = dtype
         fill_value = np.nan
diff --git a/xarray/core/duck_array_ops.py b/xarray/core/duck_array_ops.py
index 96330a64b68..3040cd71dc5 100644
--- a/xarray/core/duck_array_ops.py
+++ b/xarray/core/duck_array_ops.py
@@ -157,6 +157,8 @@ def isnull(data):
         # float types use NaN for null
         xp = get_array_namespace(data)
         return xp.isnan(data)
+    elif xp.isdtype(scalar_type, np.str_):
+        return data == ""
     elif dtypes.isdtype(scalar_type, ("bool", "integral"), xp=xp) or (
         isinstance(scalar_type, np.dtype)
         and (