diff --git a/src/chartbackground.cpp b/src/chartbackground.cpp
index edac96f..e1e8396 100644
--- a/src/chartbackground.cpp
+++ b/src/chartbackground.cpp
@@ -83,6 +83,7 @@ void ChartBackground::paint(QPainter *painter, const QStyleOptionGraphicsItem *o
 
 int ChartBackground::roundness(qreal size) const
 {
+    if(qFuzzyIsNull(size)) return 0;
     return 100*m_diameter/int(size);
 }