diff --git a/aslprep/interfaces/cbf.py b/aslprep/interfaces/cbf.py
index e5b13b5d9..11fd383e2 100644
--- a/aslprep/interfaces/cbf.py
+++ b/aslprep/interfaces/cbf.py
@@ -285,7 +285,7 @@ def _run_interface(self, runtime):
                 metadata[field] = value
 
         self._results['metadata'] = metadata
-        self._results['m0tr'] = m0tr
+        self._results['m0tr'] = np.mean(m0tr) if m0tr is not None else None
         self._results['out_file'] = fname_presuffix(
             self.inputs.name_source,
             suffix='_DeltaMOrCBF',