diff --git a/pymath/expression.py b/pymath/expression.py
index 0e7038b..ed4043f 100644
--- a/pymath/expression.py
+++ b/pymath/expression.py
@@ -89,7 +89,8 @@ class Expression(Explicable):
         if type(exp) == str:
             expression.postfix_tokens = str2tokens(exp) 
         elif type(exp) == list:
-            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 df16a9f..a211420 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
diff --git a/pymath/polynomDeg2.py b/pymath/polynomDeg2.py
index d10ad4b..ecf88ca 100644
--- a/pymath/polynomDeg2.py
+++ b/pymath/polynomDeg2.py
@@ -156,9 +156,9 @@ class Polynom_deg2(Polynom):
     def tbl_sgn_header(self):
         """ Return header of the sign line for tkzTabLine"""
         if self.delta > 0:
-            return  "{$-\\infty$, " + str(min(self.roots())) + " , " + str( max(self.roots())) + " , $+\\infty$}"
+            return  "{$-\\infty$, $" + str(min(self.roots())) + "$ , $" + str( max(self.roots())) + "$ , $+\\infty$}"
         elif self.delta == 0:
-            return  "{$-\\infty$, " +  str(self.roots()[0])  + " , $+\\infty$}"
+            return  "{$-\\infty$, $" +  str(self.roots()[0])  + "$ , $+\\infty$}"
         else:
             return  "{$-\\infty$, $+\\infty$}"