diff --git a/mathics/format/form/outputform.py b/mathics/format/form/outputform.py
index 8f67b34ce..b6697d080 100644
--- a/mathics/format/form/outputform.py
+++ b/mathics/format/form/outputform.py
@@ -3,6 +3,15 @@
 
 OutputForm is two-dimensional keyboard-character-only output, suitable for CLI
 and text terminals.
+
+The entry point of this module is the function `render_output_form`. This function takes an expression and an evaluation object as parameters, and returns a string representing the expression in its OutputForm.
+
+To do this, `render_output_form` looks for the head of the expression, and uses its name to lookup one of the registered functions
+to process specific kind of expressions. Callback functions are registered using the decorator `@register_outputform([lookupname])`.
+
+If the callback function cannot process its argument, it raises a `_WrongFormattedExpression`, to make it know `render_output_form`
+it must use the default callback function. This default function is also called when an specific callback function is not available.
+
 """
 
 import re
@@ -247,6 +256,7 @@ def render_output_form(expr: BaseElement, evaluation: Evaluation, **kwargs):
     """
     Build a pretty-print text from an `Expression`
     """
+    lookup_name: str
     format_expr: Expression = do_format(expr, evaluation, SymbolOutputForm)  # type: ignore
 
     while format_expr.has_form("HoldForm", 1):  # type: ignore
@@ -256,7 +266,7 @@ def render_output_form(expr: BaseElement, evaluation: Evaluation, **kwargs):
         return ""
 
     head = format_expr.get_head()
-    lookup_name: str = head.get_name() or head.get_lookup_name()
+    lookup_name = head.get_name() or head.get_lookup_name()
     callback = EXPR_TO_OUTPUTFORM_TEXT_MAP.get(lookup_name, None)
     if callback is None:
         if head in evaluation.definitions.outputforms:
@@ -841,9 +851,14 @@ def _slotsequence_outputform_text(expr: Expression, evaluation: Evaluation, **kw
 
 
 @register_outputform("System`String")
-def string_render_output_form(expr: String, evaluation: Evaluation, **kwargs) -> str:
+def string_render_output_form(
+    expr: BaseElement, evaluation: Evaluation, **kwargs
+) -> str:
     from mathics.format.render.text import string as render_string
 
+    if not isinstance(expr, String):
+        raise _WrongFormattedExpression
+
     # To render a string in OutputForm, we use the
     # function that render strings from Boxed expressions.
     # When a String object is converted into Boxes,