diff --git a/twined/twine.py b/twined/twine.py
index ea0fdf8..081b026 100644
--- a/twined/twine.py
+++ b/twined/twine.py
@@ -49,7 +49,7 @@ def __init__(self, **kwargs):
             setattr(self, name, strand)
 
         self._available_strands = set(trim_suffix(name, "_schema") for name in vars(self))
-        self._available_manifest_strands = set(set(self._available_strands) & set(MANIFEST_STRANDS))
+        self._available_manifest_strands = self._available_strands & set(MANIFEST_STRANDS)
 
     def _load_twine(self, source=None):
         """Load twine from a *.json filename, file-like or a json string and validates twine contents."""
@@ -214,7 +214,7 @@ def _validate_all_expected_datasets_are_present_in_manifest(self, manifest_kind,
     def available_strands(self):
         """Get the names of strands that are found in this twine.
 
-        :return tuple:
+        :return set:
         """
         return self._available_strands
 
@@ -222,7 +222,7 @@ def available_strands(self):
     def available_manifest_strands(self):
         """Get the names of the manifest strands that are found in this twine.
 
-        :return tuple:
+        :return set:
         """
         return self._available_manifest_strands