diff --git a/propka/hybrid36.py b/propka/hybrid36.py
index e0af2dc..67f2c0c 100644
--- a/propka/hybrid36.py
+++ b/propka/hybrid36.py
@@ -4,16 +4,21 @@ http://cci.lbl.gov/hybrid_36/
 """
 import string
 
-_hybrid36_upper_chars = set(string.ascii_uppercase)
-_hybrid36_lower_chars = set(string.ascii_lowercase)
-_hybrid36_digits = set(string.digits)
-_hybrid36_upper_set = _hybrid36_upper_chars | _hybrid36_digits
-_hybrid36_lower_set = _hybrid36_lower_chars | _hybrid36_digits
+
+_HYBRID36_UPPER_CHARS = set(string.ascii_uppercase)
+_HYBRID36_LOWER_CHARS = set(string.ascii_lowercase)
+_HYBRID36_DIGITS = set(string.digits)
+_HYBRID36_UPPER_SET = _HYBRID36_UPPER_CHARS | _HYBRID36_DIGITS
+_HYBRID36_LOWER_SET = _HYBRID36_LOWER_CHARS | _HYBRID36_DIGITS
+
 
 def decode(input_string):
-    """
-    Convert an input string of a number in hybrid-36 format to an integer.
+    """Convert an input string of a number in hybrid-36 format to an integer.
 
+    Args:
+        input_string:  input string
+    Returns:
+        integer
     """
     value_error_message = "invalid literal for hybrid-36 conversion: '%s'"
 
@@ -27,7 +32,7 @@ def decode(input_string):
     else:
         sign = 1
 
-    if not len(input_string):
+    if len(input_string) == 0:
         raise ValueError(value_error_message % input_string)
 
     # See http://cci.lbl.gov/hybrid_36/ for documentation on the format.
@@ -35,21 +40,21 @@ def decode(input_string):
     num_chars = len(input_string)
     first_char = input_string[0]
 
-    if first_char in _hybrid36_digits:
+    if first_char in _HYBRID36_DIGITS:
         return sign * int(input_string)
-    elif first_char in _hybrid36_upper_chars:
+    elif first_char in _HYBRID36_UPPER_CHARS:
         reference = - (10 * 36 ** (num_chars - 1) - 10 ** num_chars)
-        _hybrid36_set = _hybrid36_upper_set
-    elif first_char in _hybrid36_lower_chars:
+        _hybrid36_set = _HYBRID36_UPPER_SET
+    elif first_char in _HYBRID36_LOWER_CHARS:
         reference = (16 * 36 ** (num_chars - 1) + 10 ** num_chars)
-        _hybrid36_set = _hybrid36_lower_set
+        _hybrid36_set = _HYBRID36_LOWER_SET
     else:
         raise ValueError(value_error_message % original_input_string)
 
     # Check the validity of the input string: ASCII characters should be
     # either all uppercase or all lowercase.
-    for c in input_string[1:]:
-        if c not in _hybrid36_set:
+    for char in input_string[1:]:
+        if char not in _hybrid36_set:
             raise ValueError(value_error_message % original_input_string)
 
     # Convert with the int function.
diff --git a/tests/test_hybrid36.py b/tests/test_hybrid36.py
new file mode 100644
index 0000000..1af74d0
--- /dev/null
+++ b/tests/test_hybrid36.py
@@ -0,0 +1,57 @@
+import unittest
+
+import propka.hybrid36 as hybrid36
+
+class Hybrid36Test(unittest.TestCase):
+    def testDecode(self):
+        test_values = {
+            "99999": 99999,
+            "A0000": 100000,
+            "0": 0,
+            "9": 9,
+            "A": 10,
+            "  ZZZZY": 43770014,
+            "ZZZZZ": 43770015, # ZZZZZ - A0000 + 100000
+            "a0000": 43770016,
+            "zzzzz": 87440031,
+            "zzzzy": 87440030,
+            "99": 99,
+            "A0": 100,
+            "ZZ": 1035,
+            "zz": 1971,
+            "-99999": -99999,
+            "-A0000": -100000,
+            "-0": 0,
+            "-9": -9,
+            "-A": -10,
+            "-ZZZZY": -43770014,
+            "-ZZZZZ": -43770015, # ZZZZZ - A0000 + 100000
+            "-a0000": -43770016,
+            "-zzzzz": -87440031,
+            "-zzzzy": -87440030,
+            "-99": -99,
+            "-A0": -100,
+            "-ZZ": -1035,
+            "-zz": -1971,
+            "PROPKA": 954495146,
+            "A001Z": 100071,
+            "B0000": 1779616,
+        }
+
+        for k, v in test_values.items():
+            self.assertEqual(hybrid36.decode(k), v)
+
+    def testErrors(self):
+        test_values = [
+            "99X99",
+            "X9-99",
+            "XYZa",
+            "",
+            "-",
+            "!NotOk",
+        ]
+
+        for v in test_values:
+            with self.assertRaises(ValueError) as e:
+                hybrid36.decode(v)
+            self.assertTrue(v in str(e.exception))
\ No newline at end of file