diff --git a/pyEvalData/evaluation.py b/pyEvalData/evaluation.py
index da22dc5..52a6382 100644
--- a/pyEvalData/evaluation.py
+++ b/pyEvalData/evaluation.py
@@ -75,7 +75,7 @@ def __init__(self, source):
         self.custom_counters = []
         self.math_keys = ['mean', 'sum', 'diff', 'max', 'min', 'round', 'abs',
                           'sin', 'cos', 'tan', 'arcsin', 'arccos', 'arctan',
-                          'pi', 'exp', 'log', 'log10', 'sqrt']
+                          'pi', 'exp', 'log', 'log10', 'sqrt', 'sign']
         self.statistic_type = 'gauss'
         self.propagate_errors = True
         self.apply_data_filter = False