diff --git a/peartree/summarizer.py b/peartree/summarizer.py
index 72e380d..ed2b9d2 100644
--- a/peartree/summarizer.py
+++ b/peartree/summarizer.py
@@ -147,7 +147,7 @@ def generate_summary_wait_times(
     df_sub = df[['stop_id',
                  'wait_dir_0',
                  'wait_dir_1']].reset_index(drop=True)
-    init_of_stop_ids = df_sub.stop_id.unique()
+    init_of_stop_ids = df_sub['stop_id'].unique()
 
     # Default values for average waits with not enough data should be
     # NaN types, but let's make sure all null types are NaNs to be safe
@@ -157,7 +157,8 @@ def generate_summary_wait_times(
 
         # Convert anything that is 0 or less seconds to a NaN as well
         # to remove negative or 0 second waits in the system
-        df_sub.loc[~(df_sub[col] > 0), col] = np.nan
+        over_zero_mask = df_sub[col] > 0
+        df_sub.loc[~over_zero_mask, col] = np.nan
 
         # With all null types converted to NaN, we can cast col as float
         df_sub[col] = df_sub[col].astype(float)
diff --git a/tests/test_summarizer.py b/tests/test_summarizer.py
new file mode 100644
index 0000000..10c2f4e
--- /dev/null
+++ b/tests/test_summarizer.py
@@ -0,0 +1,35 @@
+import numpy as np
+import pandas as pd
+from peartree.summarizer import generate_summary_wait_times
+
+
+def test_generate_summary_wait_times():
+    df = pd.DataFrame({
+        'stop_id': [
+            1,
+            1,
+            2,
+            2,
+            3,
+            4],
+        'wait_dir_0': [
+            10,
+            10,
+            19,
+            21,
+            np.nan,
+            12],
+        'wait_dir_1': [
+            np.nan,
+            np.nan,
+            9,
+            11,
+            np.nan,
+            np.nan],
+    })
+
+    fallback_stop_cost = 40.0  # seconds
+    res = generate_summary_wait_times(df, fallback_stop_cost)
+    res = res.sort_values(by='stop_id')
+    assert res['stop_id'].tolist() == [1, 2, 3, 4]
+    assert res['avg_cost'].tolist() == [10.0, 15.0, 40.0, 12.0]