diff --git a/mapytex/calculus/core/MO/mo.py b/mapytex/calculus/core/MO/mo.py
index b3637f3..b15242f 100644
--- a/mapytex/calculus/core/MO/mo.py
+++ b/mapytex/calculus/core/MO/mo.py
@@ -67,7 +67,22 @@ class MO(object):
 
     @classmethod
     def factory(cls, value):
-        """ Factory to ensure that a value is a MO before using it """
+        """ Factory to ensure that a value is a MO before using it 
+
+        Idempotent??
+
+        >>> MO.factory("x")
+        <MOstr x>
+        >>> MO.factory(2)
+        <MOnumber 2>
+        >>> MO.factory(2.3)
+        <MOnumber 2.29999999999999982236431605997495353221893310546875>
+        >>> MO.factory(Decimal("2.3"))
+        <MOnumber 2.3>
+        >>> x = MO.factory("x")
+        >>> MO.factory(x)
+        <MOstr x>
+        """
         if isinstance(value, str):
             return MOstr(value)
         elif isinstance(value, int) \
@@ -77,7 +92,8 @@ class MO(object):
         elif isinstance(value, MO):
             return value
 
-        return MO(value)
+        raise MOError("Can't convert it into a MO."
+                f"Need str, int, Decimal, float or MO, got {value}")
 
     def __repr__(self):
         return f"<{self.__class__.__name__} {self.__txt__}>"