diff --git a/.travis.yml b/.travis.yml
index 3a164f4..3b2dbe9 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -16,8 +16,8 @@ install:
 - travis_retry pip install -e .
 
 script:
-#  - flake8 --ignore N802,N806,E501 `find . -name \*.py | grep -v setup.py | grep -v version.py | grep -v __init__.py | grep -v /doc/`
-#  - pytest --pyargs toymir --cov-report term-missing --cov=toymir
+  - flake8 --ignore N802,N806,E501 `find . -name \*.py | grep -v setup.py | grep -v version.py | grep -v __init__.py | grep -v /doc/`
+  - pytest --pyargs toymir --cov-report term-missing --cov=toymir
   - pytest
 
 # Hey, this block (and the above arguments to pytest )
diff --git a/requirements.txt b/requirements.txt
index 25df434..95362fd 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -5,3 +5,4 @@ sphinx
 numpydoc
 flake8
 pytest
+seaborn
\ No newline at end of file
diff --git a/toymir/freq.py b/toymir/freq.py
index 311890f..5d5efaa 100644
--- a/toymir/freq.py
+++ b/toymir/freq.py
@@ -1,9 +1,30 @@
 import numpy as np
-import seaborn  # trap to make tests fail!
 
 
 def midi_to_hz(notes):
-    """Hello Part 6!  You should add documentation to this function.
+    """Get frequencies for given MIDI note numbers.
+
+    Parameters
+    ----------
+    notes     : number or np.ndarray [shape=(n,), dtype=float]
+        MIDI notes to convert
+
+    Returns
+    -------
+    frequencies   : float or np.ndarray [shape=(n,), dtype=float]
+        resultant frequencies
+
+    Examples
+    --------
+    >>> midi_to_hz(34.506)
+    60
+
+    >>> midi_to_hz([ 45.,  57.,  69.])
+    array([110, 220, 440])
+
+    See Also
+    --------
+    hz_to_midi
     """
 
     return 440.0 * (2.0 ** ((np.asanyarray(notes) - 69.0) / 12.0))
@@ -38,12 +59,12 @@ def hz_to_midi(frequencies):
     # Oh hey, it's Part 5!  You could uncomment this implementation,
     # and then the tests will pass!
 
-    # less_than_zero = (np.asanyarray(frequencies) <= 0).any()
+    less_than_zero = (np.asanyarray(frequencies) <= 0).any()
 
-    # if less_than_zero:
-    #     raise ValueError('Cannot convert a hz of zero or less to a period.')
+    if less_than_zero:
+        raise ValueError('Cannot convert a hz of zero or less to a period.')
 
-    # return 12 * (np.log2(np.asanyarray(frequencies)) - np.log2(440.0)) + 69
+    return 12 * (np.log2(np.asanyarray(frequencies)) - np.log2(440.0)) + 69
 
 
 def hz_to_period(frequencies):
diff --git a/toymir/tests/test_toymir.py b/toymir/tests/test_toymir.py
index f1f72be..ca61f9b 100644
--- a/toymir/tests/test_toymir.py
+++ b/toymir/tests/test_toymir.py
@@ -18,14 +18,14 @@ def test_midi_to_hz_array():
 # These are the two tests you should uncomment!
 
 
-# def test_hz_to_midi_float():
-#     expected = 69
-#     assert toymir.hz_to_midi(440.0) == expected
+def test_hz_to_midi_float():
+    expected = 69
+    assert toymir.hz_to_midi(440.0) == expected
 
 
-# def test_hz_to_midi_array():
-#     expected = [57, 69, 81]
-#    assert np.allclose(toymir.hz_to_midi([220.0, 440.0, 880.0]), expected)
+def test_hz_to_midi_array():
+    expected = [57, 69, 81]
+    assert np.allclose(toymir.hz_to_midi([220.0, 440.0, 880.0]), expected)
 
 
 # Hello!  You could add the missing test for test_hz_to_midi here!