--- a/gfx/layers/BSPTree.h
+++ b/gfx/layers/BSPTree.h
@@ -15,17 +15,17 @@
 
 namespace mozilla {
 namespace layers {
 
 // Represents a node in a BSP tree. The node contains at least one polygon that
 // is used as a splitting plane, and at most two child nodes that represent the
 // splitting planes that further subdivide the space.
 struct BSPTreeNode {
-  BSPTreeNode(const gfx::Polygon3D& aPolygon)
+  explicit BSPTreeNode(const gfx::Polygon3D& aPolygon)
   {
     AddPolygon(aPolygon);
   }
 
   void AddPolygon(const gfx::Polygon3D& aPolygon)
   {
     polygons.AppendElement(aPolygon);
   }