diff --git a/tests/testthat/test_themes.R b/tests/testthat/test_themes.R
index a3c3898..63a3f2e 100644
--- a/tests/testthat/test_themes.R
+++ b/tests/testthat/test_themes.R
@@ -26,7 +26,9 @@
     capture.output({
       output_file <- tempfile(fileext = ".html")
       output_format <- revealjs_presentation(theme = theme)
-      rmarkdown::render(testdoc, output_file = output_file)
+      rmarkdown::render(testdoc, 
+                        output_format = output_format,
+                        output_file = output_file)
       expect_true(file.exists(output_file))
     })
   })