diff --git a/pygal/graph/bar.py b/pygal/graph/bar.py
index 2f3cf5c..2bd352b 100644
--- a/pygal/graph/bar.py
+++ b/pygal/graph/bar.py
@@ -106,12 +106,12 @@ class Bar(Graph):
 
     def _compute_secondary(self):
         if self.secondary_series:
-            y_pos = zip(*self._y_labels)[1]
+            y_pos = list(zip(*self._y_labels))[1]
             ymin = self._secondary_min
             ymax = self._secondary_max
 
-            min_0_ratio = (self.zero - self._box.ymin) / self._box.height
-            max_0_ratio = (self._box.ymax - self.zero) / self._box.height
+            min_0_ratio = (self.zero - self._box.ymin) / self._box.height or 1
+            max_0_ratio = (self._box.ymax - self.zero) / self._box.height or 1
 
             new_ymax = (self.zero - ymin) * (1 / min_0_ratio - 1)
             new_ymin = -(ymax - self.zero) * (1 / max_0_ratio - 1)
diff --git a/pygal/graph/graph.py b/pygal/graph/graph.py
index f77afd3..8ecc857 100644
--- a/pygal/graph/graph.py
+++ b/pygal/graph/graph.py
@@ -463,7 +463,7 @@ class Graph(BaseGraph):
     def _compute_secondary(self):
         # secondary y axis support
         if self.secondary_series and self._y_labels:
-            y_pos = zip(*self._y_labels)[1]
+            y_pos = list(zip(*self._y_labels))[1]
             if self.include_x_axis:
                 ymin = min(self._secondary_min, 0)
                 ymax = max(self._secondary_max, 0)
diff --git a/pygal/graph/stackedbar.py b/pygal/graph/stackedbar.py
index bb8a3c1..610eacb 100644
--- a/pygal/graph/stackedbar.py
+++ b/pygal/graph/stackedbar.py
@@ -85,10 +85,10 @@ class StackedBar(Bar):
 
             # In case of pyramids
             sum_ = lambda x: sum(x) if isinstance(x, tuple) else x
-            self._secondary_min = negative_vals and min(
-                sum_(min(negative_vals)), self.zero)
-            self._secondary_max = positive_vals and max(
-                sum_(max(positive_vals)), self.zero)
+            self._secondary_min = (negative_vals and min(
+                sum_(min(negative_vals)), self.zero)) or self.zero
+            self._secondary_max = (positive_vals and max(
+                sum_(max(positive_vals)), self.zero)) or self.zero
 
     def _bar(self, parent, x, y, index, i, zero, shift=False, secondary=False):
         if secondary:
diff --git a/pygal/graph/verticalpyramid.py b/pygal/graph/verticalpyramid.py
index 308fb76..7afb92f 100644
--- a/pygal/graph/verticalpyramid.py
+++ b/pygal/graph/verticalpyramid.py
@@ -54,7 +54,7 @@ class VerticalPyramid(StackedBar):
     def _compute_secondary(self):
         # Need refactoring
         if self.secondary_series:
-            y_pos = zip(*self._y_labels)[1]
+            y_pos = list(zip(*self._y_labels))[1]
             positive_vals, negative_vals = self._get_separated_values(True)
             positive_sum = map(sum, positive_vals) or [self.zero]
             negative_sum = map(sum, negative_vals) or [self.zero]