def METHOD_NAME(node, cstruct):
    """Validate that a ``colander.Mapping`` only has strings in its values.
    .. warning::
        Should be associated to a ``colander.Mapping`` schema node.
    """
    are_strings = [isinstance(v, str) for v in cstruct.values()]
    if not all(are_strings):
        error_msg = f"{cstruct} contains non string value"
        raise colander.Invalid(node, error_msg)