diff --git a/plotly/tools.py b/plotly/tools.py
index 6d5de58e3be..670641ce40a 100644
--- a/plotly/tools.py
+++ b/plotly/tools.py
@@ -1693,7 +1693,7 @@ def _validate_gantt(df):
                 if key not in df:
                     raise exceptions.PlotlyError(
                         "The columns in your dataframe must include the "
-                        "keys".format(REQUIRED_GANTT_KEYS)
+                        "following keys: {0}".format(', '.join(REQUIRED_GANTT_KEYS))
                     )
 
             num_of_rows = len(df.index)