diff --git a/nipype/interfaces/afni/tests/test_auto_CatMatvec.py b/nipype/interfaces/afni/tests/test_auto_CatMatvec.py
index 4b79cd91d8..d3d94569be 100644
--- a/nipype/interfaces/afni/tests/test_auto_CatMatvec.py
+++ b/nipype/interfaces/afni/tests/test_auto_CatMatvec.py
@@ -23,11 +23,11 @@ def test_CatMatvec_inputs():
     ),
     matrix=dict(argstr='-MATRIX',
     descr="indicates that the resulting matrix willbe written to outfile in the 'MATRIX(...)' format (FORM 3).This feature could be used, with clever scripting, to inputa matrix directly on the command line to program 3dWarp.",
-    xor=['oneline', 'fourXfour'],
+    xor=['oneline', 'fourxfour'],
     ),
     oneline=dict(argstr='-ONELINE',
     descr='indicates that the resulting matrixwill simply be written as 12 numbers on one line.',
-    xor=['matrix', 'fourXfour'],
+    xor=['matrix', 'fourxfour'],
     ),
     out_file=dict(argstr=' > %s',
     descr='File to write concattenated matvecs to',
diff --git a/nipype/interfaces/afni/utils.py b/nipype/interfaces/afni/utils.py
index e20fe1d5ff..88a317b8ce 100644
--- a/nipype/interfaces/afni/utils.py
+++ b/nipype/interfaces/afni/utils.py
@@ -602,12 +602,12 @@ class CatMatvecInputSpec(AFNICommandInputSpec):
                "This feature could be used, with clever scripting, to input"
                "a matrix directly on the command line to program 3dWarp.",
         argstr="-MATRIX",
-        xor=['oneline','fourXfour'])
+        xor=['oneline', 'fourxfour'])
     oneline = traits.Bool(
         descr="indicates that the resulting matrix"
               "will simply be written as 12 numbers on one line.",
         argstr="-ONELINE",
-        xor=['matrix','fourXfour'])
+        xor=['matrix', 'fourxfour'])
     fourxfour = traits.Bool(
         descr="Output matrix in augmented form (last row is 0 0 0 1)"
               "This option does not work with -MATRIX or -ONELINE",