diff --git a/stingray/base.py b/stingray/base.py
index a1c5474f5..ce0b5c04e 100644
--- a/stingray/base.py
+++ b/stingray/base.py
@@ -511,7 +511,7 @@ def write(self, filename: str, fmt: str = None) -> None:
             os.unlink(probe_file)
             raise ValueError("float128")
         except ValueError as e:
-            if not "float128" in str(e):  # pragma: no cover
+            if "float128" not in str(e):  # pragma: no cover
                 raise
             warnings.warn(
                 f"{fmt} output does not allow saving metadata at maximum precision. "
@@ -525,7 +525,7 @@ def write(self, filename: str, fmt: str = None) -> None:
             os.unlink(probe_file)
             CAN_SERIALIZE_META = True
         except TypeError as e:
-            if not "serialize_meta" in str(e):
+            if "serialize_meta" not in str(e):
                 raise
             warnings.warn(
                 f"{fmt} output does not serialize the metadata at the moment. "