diff --git a/boxmox/data.py b/boxmox/data.py
index 8327b1307a0f75e08cadfbf9928da43df642afd3..256e225d00d9468ba01d9ab4da6c8a8cba7c94fb 100644
--- a/boxmox/data.py
+++ b/boxmox/data.py
@@ -141,10 +141,10 @@ class InputFile17:
                 f.write(' '.join('{0:e}'.format(x) for x in line) + '\n')
         else:
             def _data_fix(key):
-                if isinstance(self._data[x], list):
-                    return self._data[x][0]
+                if isinstance(self._data[key], list):
+                    return self._data[key][0]
                 else:
-                    return self._data[x]
+                    return self._data[key]
             data_line = ' '.join( '{0:e}'.format( float(self.anc[x])) for x in self.anc.keys() ) + ' ' + ' '.join( '{0:e}'.format( float(_data_fix(x))) for x in self.keys() ) + '\n'
             if not self.time is None:
                 data_line = '{0:e}'.format(float(self.time)) + ' ' + data_line
@@ -233,10 +233,10 @@ class InputFileOrig:
         else:
             f.write('{0:s}' .format(' '.join( column_names)) +'\n')
             def _data_fix(key):
-                if isinstance(self._data[x], list):
-                    return self._data[x][0]
+                if isinstance(self._data[key], list):
+                    return self._data[key][0]
                 else:
-                    return self._data[x]
+                    return self._data[key]
             f.write(' '.join( '{0:e}'.format(float(_data_fix(x))) for x in self.keys() ) + '\n')
 
     def __init__(self, fpath=None):