diff --git a/nipype/interfaces/mrtrix3/preprocess.py b/nipype/interfaces/mrtrix3/preprocess.py
index be87930a38..a097295eea 100644
--- a/nipype/interfaces/mrtrix3/preprocess.py
+++ b/nipype/interfaces/mrtrix3/preprocess.py
@@ -257,7 +257,8 @@ def _format_arg(self, name, trait_spec, value):
 
     def _list_outputs(self):
         outputs = self.output_spec().get()
-        outputs["out_file"] = op.abspath(self.inputs.out_file)
+        if self.inputs.out_file:
+            outputs["out_file"] = op.abspath(self.inputs.out_file)
         if self.inputs.bias:
             outputs["bias"] = op.abspath(self.inputs.bias)
         return outputs