diff --git a/xarray/core/dataset.py b/xarray/core/dataset.py
index 3ca222b1965..9c2c2f60db1 100644
--- a/xarray/core/dataset.py
+++ b/xarray/core/dataset.py
@@ -5433,7 +5433,7 @@ def _unstack_once(
             if name not in index_vars:
                 if dim in var.dims:
                     if isinstance(fill_value, Mapping):
-                        fill_value_ = fill_value[name]
+                        fill_value_ = fill_value.get(name, xrdtypes.NA)
                     else:
                         fill_value_ = fill_value
 
diff --git a/xarray/tests/test_dataset.py b/xarray/tests/test_dataset.py
index 630bfea46c5..e677430dfbf 100644
--- a/xarray/tests/test_dataset.py
+++ b/xarray/tests/test_dataset.py
@@ -4150,6 +4150,10 @@ def test_unstack_fill_value(self) -> None:
         expected3 = ds.unstack("index").fillna({"var": -1, "other_var": 1}).astype(int)
         assert_equal(actual3, expected3)
 
+        actual4 = ds.unstack("index", fill_value={"var": -1})
+        expected4 = ds.unstack("index").fillna({"var": -1, "other_var": np.nan})
+        assert_equal(actual4, expected4)
+
     @requires_sparse
     def test_unstack_sparse(self) -> None:
         ds = xr.Dataset(