diff --git a/sped/campos.py b/sped/campos.py
index fdebc83..fe524cc 100644
--- a/sped/campos.py
+++ b/sped/campos.py
@@ -57,7 +57,7 @@ def set(self, registro, valor):
             return
         if valor and not self.__class__.validar(valor):
             raise FormatoInvalidoError(registro, self.nome)
-        if not isinstance(valor, str):
+        if not isinstance(valor, basestring):
             raise FormatoInvalidoError(registro, self.nome)
         registro.valores[self._indice] = valor or ''