diff --git a/pydantic/types.py b/pydantic/types.py
index fb5fa39..8918b7d 100644
--- a/pydantic/types.py
+++ b/pydantic/types.py
@@ -56,14 +56,14 @@ class ConstrainedStr(str):
         if value is None:
             raise TypeError('None is not an allow value')
 
-        l = len(value)
-        if cls.min_length is not None and l < cls.min_length:
+        v_len = len(value)
+        if cls.min_length is not None and v_len < cls.min_length:
             raise ValueError(f'length less than minimum allowed: {cls.min_length}')
 
         if cls.curtail_length:
-            if l > cls.curtail_length:
+            if v_len > cls.curtail_length:
                 value = value[:cls.curtail_length]
-        elif cls.max_length is not None and l > cls.max_length:
+        elif cls.max_length is not None and v_len > cls.max_length:
             raise ValueError(f'length greater than maximum allowed: {cls.max_length}')
 
         if cls.regex:
diff --git a/tests/requirements.txt b/tests/requirements.txt
index 2153cff..44ca562 100644
--- a/tests/requirements.txt
+++ b/tests/requirements.txt
@@ -1,5 +1,5 @@
 coverage==4.4.1
-flake8==3.4.1
+flake8==3.5.0
 isort==4.2.15
 mypy==0.540
 pycodestyle==2.3.1