diff --git a/nipype/interfaces/ants/resampling.py b/nipype/interfaces/ants/resampling.py
index 39393dc0f0..92d84b15b2 100644
--- a/nipype/interfaces/ants/resampling.py
+++ b/nipype/interfaces/ants/resampling.py
@@ -195,7 +195,7 @@ def _format_arg(self, opt, spec, val):
                         isdefined(self.inputs.invert_affine):
                     affine_counter += 1
                     if affine_counter in self.inputs.invert_affine:
-                        series += '-i',
+                        series += ['-i'],
                 series += [transformation]
             return ' '.join(series)
         return super(WarpImageMultiTransform, self)._format_arg(opt, spec, val)