diff --git a/PQAnalysis/io/traj_file/frame_reader.py b/PQAnalysis/io/traj_file/frame_reader.py
index 1a615ce..ebdb00d 100644
--- a/PQAnalysis/io/traj_file/frame_reader.py
+++ b/PQAnalysis/io/traj_file/frame_reader.py
@@ -251,7 +251,9 @@ def _check_qmcfc(
         return value, atoms
 
     def _get_topology(
-        self, atoms: List[str], topology: Topology | None
+        self,
+        atoms: List[str],
+        topology: Topology | None,
     ) -> Topology:
         """
         Returns the topology of the frame.
diff --git a/PQAnalysis/io/traj_file/trajectory_reader.py b/PQAnalysis/io/traj_file/trajectory_reader.py
index 2829aa8..e1fa84b 100644
--- a/PQAnalysis/io/traj_file/trajectory_reader.py
+++ b/PQAnalysis/io/traj_file/trajectory_reader.py
@@ -252,7 +252,7 @@ def frame_generator(
                             # then increment the frame index
                             frame_index += 1
 
-                            if self.constant_topology and self.topology is not None:
+                            if self.constant_topology and self.topology is None:
                                 self.topology = frame.topology
 
                         frame_lines = [line]
@@ -277,7 +277,7 @@ def frame_generator(
                     # then increment the frame index
                     frame_index += 1
 
-                if self.constant_topology and self.topology is not None:
+                if self.constant_topology and self.topology is None:
                     self.topology = frame.topology
 
     @runtime_type_checking