diff --git a/mapytex/calculus/core/renders/tree2tex.py b/mapytex/calculus/core/renders/tree2tex.py
index 84d41d5..651912c 100644
--- a/mapytex/calculus/core/renders/tree2tex.py
+++ b/mapytex/calculus/core/renders/tree2tex.py
@@ -29,23 +29,13 @@ def plus2tex(left, right):
     >>> plus2tex(MO.factory(3), t)
     '3 - 2 \\times 3'
     """
-    display_plus = True
-    try:
-        left.node
-    except AttributeError:
-        left_ = left.__tex__
-    else:
-        left_ = tree2tex(left)
 
-    try:
-        right.node
-    except AttributeError:
-        right_ = right.__tex__
-    else:
-        right_ = tree2tex(right)
-    finally:
-        if right_.startswith("-"):
-            display_plus = False
+    left_ = render_with_parenthesis(left, "+")
+    right_ = render_with_parenthesis(right, "+")
+
+    display_plus = True
+    if right_.startswith("-"):
+        display_plus = False
 
     if display_plus:
         return f"{left_} + {right_}"
@@ -97,6 +87,8 @@ def mul2tex(left, right):
     >>> a = MO.factory('x')
     >>> mul2tex(MO.factory(3), a)
     '3x'
+    >>> mul2tex(MO.factory(-3), a)
+    '- 3x'
     >>> mul2tex(a, a)
     'x \\times x'
     """
@@ -104,9 +96,15 @@ def mul2tex(left, right):
     right_ = render_with_parenthesis(right, "*")
 
     display_time = True
-    if (right_[0].isalpha() and (left_.isnumeric() or left_.isdecimal())) or right_[
-        0
-    ] == "(":
+    # if (right_[0].isalpha() and (left_.isnumeric() or left_.isdecimal())) or right_[
+    #     0
+    # ] == "(":
+    #     display_time = False
+    if right_[0].isalpha():
+        # TODO: C'est bien beurk en dessous... |ven. déc. 21 12:03:07 CET 2018
+        if type(left).__name__ == "MOnumber":
+            display_time = False
+    elif right_[0] == "(":
         display_time = False
 
     if display_time:
@@ -133,11 +131,11 @@ def div2tex(left, right):
     """
     try:
         left_ = tree2tex(left)
-    except AttributeError:
+    except (AttributeError, ValueError):
         left_ = left.__tex__
     try:
         right_ = tree2tex(right)
-    except AttributeError:
+    except (AttributeError, ValueError):
         right_ = right.__tex__
 
     return "\\frac{" + left_ + "}{" + right_ + "}"
@@ -226,6 +224,10 @@ def tree2tex(tree):
     >>> tree2tex(t)
     '2 + 3 \\times 4'
     """
+    from ..tree import Tree
+
+    if not isinstance(tree, Tree):
+        raise ValueError(f"Can only render a Tree (got {type(tree).__name__}: {tree})")
     return OPERATOR2TEX[tree.node](tree.left_value, tree.right_value)