diff --git a/leather/shapes/grouped_bars.py b/leather/shapes/grouped_bars.py
index 0a32716..40ea6b8 100644
--- a/leather/shapes/grouped_bars.py
+++ b/leather/shapes/grouped_bars.py
@@ -15,8 +15,8 @@ class GroupedBars(CategoryShape):
     Render a categorized series of data as grouped bars.
 
     :param fill_color:
-        A sequence of colors to fill the bars. If the sequence is shorter than
-        the number of values in any category, the colors will be repeated.
+        A sequence of colors to fill the bars. The sequence must have length
+        greater than or equal to the number of values in any category.
     """
     def __init__(self, fill_color=None):
         self._fill_color = fill_color
@@ -26,8 +26,11 @@ def validate_series(self, series):
         """
         Verify this shape can be used to render a given series.
         """
-        if issequence(self._fill_color) and len(series.categories()) > len(self._fill_color.keys()):
-            raise ValueError('fill_color must have an element for every category in the series.')
+        if len(series.categories()) > len(self._fill_color):
+            raise ValueError('Fill color must have an element for every category in the series.')
+
+        if isinstance(series, CategorySeries):
+            raise ValueError('GroupedBars can only be used to render CategorySeries.')
 
     def to_svg(self, width, height, x_scale, y_scale, series, palette):
         """
@@ -43,6 +46,9 @@ def to_svg(self, width, height, x_scale, y_scale, series, palette):
         else:
             fill_color = list(palette)
 
+            if len(series.categories()) > len(fill_color):
+                raise ValueError('Fill color must have an element for every category in the series.')
+
         label_colors = self.legend_labels(series, fill_color)
 
         categories = series.categories()
diff --git a/tests/test_shapes.py b/tests/test_shapes.py
index 2c83937..e1d25eb 100644
--- a/tests/test_shapes.py
+++ b/tests/test_shapes.py
@@ -195,6 +195,12 @@ def test_to_svg(self):
         self.assertEqual(float(rects[1].get('width')), 100)
         self.assertEqual(rects[1].get('fill'), 'white')
 
+    def test_invalid_fill_color(self):
+        series = leather.CategorySeries(self.rows)
+
+        with self.assertRaises(ValueError):
+            group = self.shape.to_svg(200, 100, self.linear, self.ordinal, series, ['one', 'two'])
+
     def test_nulls(self):
         series = leather.CategorySeries([
             (0, 'foo', 'first'),