diff --git a/maize/steps/exs/biosimspace/_base.py b/maize/steps/exs/biosimspace/_base.py
index bdde1d6..48a468f 100644
--- a/maize/steps/exs/biosimspace/_base.py
+++ b/maize/steps/exs/biosimspace/_base.py
@@ -217,7 +217,7 @@ def _dump_data(self) -> None:
                 else:
                     shutil.copy(file, dump_dir)
 
-    def _get_executable(self) -> str:
+    def _get_executable(self) -> str | None:
         """Get the full path to the executable."""
         if self.required_callables:
             return shutil.which(self.required_callables[0])