diff --git a/pymath/expression.py b/pymath/expression.py
index b54437e..ed4043f 100644
--- a/pymath/expression.py
+++ b/pymath/expression.py
@@ -89,8 +89,8 @@ class Expression(Explicable):
         if type(exp) == str:
             expression.postfix_tokens = str2tokens(exp) 
         elif type(exp) == list:
-            # Bug ici quand i ne peut pas être converti en string
-            exp_mod_op = [op.get_op(i) if (str(i) in "+-*/^" and not isOperator(i)) else i for i in exp]
+            # Ici on ne peut convertir les "+" en opérateur que s'ils sont d'arité 2.
+            exp_mod_op = [op.get_op(i) if op.can_be_operator(i) else i for i in exp]
             expression.postfix_tokens = flatten_list([tok.postfix_tokens if Expression.isExpression(tok) else tok for tok in exp_mod_op])
         elif type(exp) == Expression:
             return exp 
diff --git a/pymath/operator.py b/pymath/operator.py
index b643cad..18a142b 100644
--- a/pymath/operator.py
+++ b/pymath/operator.py
@@ -268,7 +268,10 @@ class op(object):
     @classmethod
     def can_be_operator(cls, symbole):
         """ Tell if the symbole can be an operator """
-        return symbole in [i[0] for i in cls._operators]
+        if type(symbole) == str:
+            return symbole in [i[0] for i in cls._operators]
+        else:
+            return False
 
 
     @ClassProperty