diff --git a/nipype/interfaces/afni/utils.py b/nipype/interfaces/afni/utils.py
index 7fadffe5b4..48f1303d2f 100644
--- a/nipype/interfaces/afni/utils.py
+++ b/nipype/interfaces/afni/utils.py
@@ -694,10 +694,8 @@ class CenterMassOutputSpec(TraitedSpec):
         desc='output file')
     cm_file = File(
         desc='file with the center of mass coordinates')
-    cm = traits.Either(
+    cm = traits.List(
         traits.Tuple(traits.Float(), traits.Float(), traits.Float()),
-        traits.List(traits.Tuple(traits.Float(), traits.Float(),
-                                 traits.Float())),
         desc='center of mass')
 
 
@@ -734,11 +732,8 @@ def _list_outputs(self):
         outputs = super(CenterMass, self)._list_outputs()
         outputs['out_file'] = os.path.abspath(self.inputs.in_file)
         outputs['cm_file'] = os.path.abspath(self.inputs.cm_file)
-        sout = np.loadtxt(outputs['cm_file'])  # pylint: disable=E1101
-        if len(sout) > 1:
-            outputs['cm'] = [tuple(s) for s in sout]
-        else:
-            outputs['cm'] = tuple(sout)
+        sout = np.loadtxt(outputs['cm_file'], ndmin=2)  # pylint: disable=E1101
+        outputs['cm'] = [tuple(s) for s in sout]
         return outputs