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..bd9ca27 100644
--- a/toymir/freq.py
+++ b/toymir/freq.py
@@ -38,12 +38,12 @@ 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..5dd86bd 100644
--- a/toymir/tests/test_toymir.py
+++ b/toymir/tests/test_toymir.py
@@ -18,18 +18,22 @@ 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!
 
+def test_hz_to_period_zero():
+    expected = ValueError
+    assert toymir.hz_to_midi(0) == expected
+
 
 def test_hz_to_period_float():
     expected = 0.1