diff --git a/cpt/src/main/java/org/isisaddons/wicket/wickedcharts/cpt/ui/summarychart/CollectionContentsAsSummaryChart.java b/cpt/src/main/java/org/isisaddons/wicket/wickedcharts/cpt/ui/summarychart/CollectionContentsAsSummaryChart.java
index c12832a..84a9d4a 100644
--- a/cpt/src/main/java/org/isisaddons/wicket/wickedcharts/cpt/ui/summarychart/CollectionContentsAsSummaryChart.java
+++ b/cpt/src/main/java/org/isisaddons/wicket/wickedcharts/cpt/ui/summarychart/CollectionContentsAsSummaryChart.java
@@ -160,7 +160,7 @@ private BigDecimal maxOf(BigDecimal max, final BigDecimal summaryMax) {
             ? summaryMax != null
                 ? max.max(summaryMax)
                 : max
-            : BigDecimal.ZERO;
+            : null;
     }
 
     private BigDecimal minOf(BigDecimal min, final BigDecimal summaryMin) {
@@ -168,7 +168,7 @@ private BigDecimal minOf(BigDecimal min, final BigDecimal summaryMin) {
             ? summaryMin != null
                 ? min.min(summaryMin)
                 : min
-            : BigDecimal.ZERO;
+            : null;
     }
 
     @Override