--- a/t/primary-prefix-invalid-couples.tap
+++ b/t/primary-prefix-invalid-couples.tap
@@ -186,8 +186,10 @@ grep -v 'dir.* not a legitimate directory' stderr && exit 1
 # Check that the same failures are present without the '--add-missing'
 # option.
 mv stderr stderr.old
+cat stderr.old | sort > a
 AUTOMAKE_fails -d "automake error out on mismatched prefix/primary couples"
+cat stderr | sort > b
 command_ok_ "... and with the same diagnostic of 'automake -a'" \
-            diff stderr.old stderr
+            diff a b
 
 :