diff --git a/R/geom-polygon.r b/R/geom-polygon.r
index 9b5f1bb291..f44ae4d633 100644
--- a/R/geom-polygon.r
+++ b/R/geom-polygon.r
@@ -167,7 +167,7 @@ GeomPolygon <- ggproto("GeomPolygon", Geom,
 
   },
 
-  default_aes = aes(colour = "NA", fill = "grey20", size = 0.5, linetype = 1,
+  default_aes = aes(colour = NA, fill = "grey20", size = 0.5, linetype = 1,
     alpha = NA, subgroup = NULL),
 
   handle_na = function(data, params) {