diff --git a/tools/phyloseq/phyloseq_plot_bar.xml b/tools/phyloseq/phyloseq_plot_bar.xml
index 914249e030e..e91dc9dfcaf 100644
--- a/tools/phyloseq/phyloseq_plot_bar.xml
+++ b/tools/phyloseq/phyloseq_plot_bar.xml
@@ -39,7 +39,7 @@ Rscript '${__tool_directory__}/phyloseq_plot_bar.R'
     <!-- Test 2: Missing x and fill parameters -->
     <test>
         <param name="input" value="output.phyloseq" ftype="phyloseq"/>
-        <param name="x" value=""/>
+        <param name="x" value="Number"/>
         <param name="fill" value=""/>
         <expect_exit_code>1</expect_exit_code>
     </test>