diff --git a/tests/interactive.config b/tests/interactive.config
index d472f3e65..b1b26af4e 100644
--- a/tests/interactive.config
+++ b/tests/interactive.config
@@ -4,7 +4,7 @@ set -g prompt_counter 0
 set -g prompt_counter_incr 0
 function fish_prompt
     if test $prompt_counter_incr -eq 1
-        set -g prompt_counter (expr $prompt_counter + 1)
+        set -g prompt_counter (math $prompt_counter + 1)
         set -g prompt_counter_incr 0
     end
     echo "prompt $prompt_counter>"