diff --git a/docs/configuration.rst b/docs/configuration.rst
index 9efafef..e6c13ee 100644
--- a/docs/configuration.rst
+++ b/docs/configuration.rst
@@ -272,6 +272,8 @@ Let's say we want to convert all unicode typed properties to TextAreaFields inst
             type_map = ClassMap({sa.Unicode: TextAreaField})
 
 
+In case the type_map dictionary values are not inherited from WTForm field class, they are considered callable functions. These functions will be called with the corresponding column as their only parameter.
+
 
 .. _custom_base:
 
diff --git a/tests/test_types.py b/tests/test_types.py
index 03f5950..981d5eb 100644
--- a/tests/test_types.py
+++ b/tests/test_types.py
@@ -275,3 +275,24 @@ class Meta:
 
         form = ModelTestForm()
         assert isinstance(form.test_column, TextAreaField)
+
+    def test_override_type_map_with_callable(self):
+        class ModelTest(self.base):
+            __tablename__ = 'model_test'
+            id = sa.Column(sa.Integer, primary_key=True)
+            test_column_short = sa.Column(sa.Unicode(255), nullable=False)
+            test_column_long = sa.Column(sa.Unicode(), nullable=False)
+
+        class ModelTestForm(ModelForm):
+            class Meta:
+                model = ModelTest
+                not_null_validator = None
+                type_map = ClassMap({
+                    sa.Unicode: lambda column: (
+                        StringField if column.type.length else TextAreaField
+                    )
+                })
+
+        form = ModelTestForm()
+        assert isinstance(form.test_column_short, StringField)
+        assert isinstance(form.test_column_long, TextAreaField)
diff --git a/wtforms_alchemy/generator.py b/wtforms_alchemy/generator.py
index 5ab1202..b27ec26 100644
--- a/wtforms_alchemy/generator.py
+++ b/wtforms_alchemy/generator.py
@@ -9,7 +9,13 @@
 import sqlalchemy as sa
 from sqlalchemy.orm.properties import ColumnProperty
 from sqlalchemy_utils import types
-from wtforms import BooleanField, FloatField, PasswordField, TextAreaField
+from wtforms import (
+    BooleanField,
+    Field,
+    FloatField,
+    PasswordField,
+    TextAreaField
+)
 from wtforms.widgets import CheckboxInput, TextArea
 from wtforms_components import (
     ColorField,
@@ -607,6 +613,10 @@ def get_field_class(self, column):
             check_type = column.type
 
         try:
-            return self.TYPE_MAP[check_type]
+            column_type = self.TYPE_MAP[check_type]
+            if inspect.isclass(column_type) and issubclass(column_type, Field):
+                return column_type
+            else:
+                return column_type(column)
         except KeyError:
             raise UnknownTypeException(column)