diff --git a/elasticsearch_dsl/search_base.py b/elasticsearch_dsl/search_base.py
index b54bbaec..76a9b1d9 100644
--- a/elasticsearch_dsl/search_base.py
+++ b/elasticsearch_dsl/search_base.py
@@ -37,13 +37,13 @@
 from typing_extensions import Self, TypeVar
 
 from .aggs import A, Agg, AggBase
+from .document_base import InstrumentedField
 from .exceptions import IllegalOperation
 from .query import Bool, Q, Query
 from .response import Hit, Response
 from .utils import _R, AnyUsingType, AttrDict, DslBase, recursive_to_dict
 
 if TYPE_CHECKING:
-    from .document_base import InstrumentedField
     from .field import Field, Object
 
 
@@ -714,10 +714,14 @@ def ensure_strings(
                 Dict[str, List[Union[str, "InstrumentedField"]]],
             ]
         ) -> Union[str, List[str], Dict[str, List[str]]]:
-            if isinstance(fields, list):
-                return [str(f) for f in fields]
-            elif isinstance(fields, dict):
+            if isinstance(fields, dict):
                 return {k: ensure_strings(v) for k, v in fields.items()}
+            elif not isinstance(fields, (str, InstrumentedField)):
+                # we assume that if `fields` is not a any of [dict, str,
+                # InstrumentedField] then it is an iterable of strings or
+                # InstrumentedFields, so we convert them to a plain list of
+                # strings
+                return [str(f) for f in fields]
             else:
                 return str(fields)
 
diff --git a/tests/_async/test_search.py b/tests/_async/test_search.py
index 603c3826..28e07a50 100644
--- a/tests/_async/test_search.py
+++ b/tests/_async/test_search.py
@@ -546,7 +546,7 @@ def test_source() -> None:
 
     assert {
         "_source": {"includes": ["foo.bar.*"], "excludes": ["foo.one"]}
-    } == AsyncSearch().source(includes=["foo.bar.*"], excludes=["foo.one"]).to_dict()
+    } == AsyncSearch().source(includes=["foo.bar.*"], excludes=("foo.one",)).to_dict()
 
     assert {"_source": False} == AsyncSearch().source(False).to_dict()
 
diff --git a/tests/_sync/test_search.py b/tests/_sync/test_search.py
index 638c614d..d777508f 100644
--- a/tests/_sync/test_search.py
+++ b/tests/_sync/test_search.py
@@ -546,7 +546,7 @@ def test_source() -> None:
 
     assert {
         "_source": {"includes": ["foo.bar.*"], "excludes": ["foo.one"]}
-    } == Search().source(includes=["foo.bar.*"], excludes=["foo.one"]).to_dict()
+    } == Search().source(includes=["foo.bar.*"], excludes=("foo.one",)).to_dict()
 
     assert {"_source": False} == Search().source(False).to_dict()