diff --git a/R/stat-contour.r b/R/stat-contour.r
index f23bc4681d..b07f6a5f5e 100644
--- a/R/stat-contour.r
+++ b/R/stat-contour.r
@@ -97,7 +97,7 @@ contour_lines <- function(data, breaks, complete = FALSE) {
     x = xs,
     y = ys,
     piece = pieces,
-    group = groups
+    group = factor(groups)
   ), n = length(xs))
 }
 
diff --git a/tests/testthat/test-layer.r b/tests/testthat/test-layer.r
index bcd3502413..352d41b88a 100644
--- a/tests/testthat/test-layer.r
+++ b/tests/testthat/test-layer.r
@@ -39,7 +39,7 @@ test_that("missing aesthetics trigger informative error", {
 })
 
 test_that("if an aes is mapped to a function that returns NULL, it is removed", {
-  df <- data.frame(x = 1:10)
+  df <- data_frame(x = 1:10)
   null <- function(...) NULL
   p <- cdata(ggplot(df, aes(x, null())))
   expect_identical(names(p[[1]]), c("x", "PANEL", "group"))