diff --git a/nipype/interfaces/spm/preprocess.py b/nipype/interfaces/spm/preprocess.py
index 7dd47ef46f..47c9fd77a4 100644
--- a/nipype/interfaces/spm/preprocess.py
+++ b/nipype/interfaces/spm/preprocess.py
@@ -125,8 +125,7 @@ def _list_outputs(self):
 
 class RealignInputSpec(SPMCommandInputSpec):
     in_files = InputMultiPath(
-        traits.Either(
-            traits.List(ImageFileSPM(exists=True)), ImageFileSPM(exists=True)),
+        ImageFileSPM(exists=True),
         field='data',
         mandatory=True,
         copyfile=True,