diff --git a/scripts/Diffraction/isis_powder/routines/run_details.py b/scripts/Diffraction/isis_powder/routines/run_details.py
index efe8bdf0dcf7508f1ad568d00dcc4f9d8a02dc8c..a694713bc9ddcf31ee4f5a10d7807b745dcfaa7a 100644
--- a/scripts/Diffraction/isis_powder/routines/run_details.py
+++ b/scripts/Diffraction/isis_powder/routines/run_details.py
@@ -41,7 +41,7 @@ def create_run_details_object(run_number_string, inst_settings, is_vanadium_run,
         output_run_string = run_number_string
 
     # Get the file extension if set
-    file_extension = getattr(inst_settings, "file_extension" )
+    file_extension = getattr(inst_settings, "file_extension")
     if file_extension:
         # Prefix dot if user has forgotten to
         file_extension = file_extension if file_extension.startswith('.') else '.' + file_extension
@@ -183,6 +183,6 @@ class _RunDetails(object):
         self.vanadium_run_numbers = vanadium_run_number
 
         # Optional
-        self.file_extension = str(file_extension)
+        self.file_extension = str(file_extension) if file_extension else None
         self.sample_empty = sample_empty
         self.vanadium_absorption_path = vanadium_abs_path