diff --git a/tests/30backend-threadpool.tests b/tests/30backend-threadpool.tests
index 6ba717091..65125489d 100644
--- a/tests/30backend-threadpool.tests
+++ b/tests/30backend-threadpool.tests
@@ -10,7 +10,7 @@ END
 
 TEST "process a function" process-function <<END
 err: the input to the function is
-err: finished waiting
+err: the result is 1
 END
 
 PLANNED "error cleared on join"
diff --git a/tests/backend/test-threadpool.c b/tests/backend/test-threadpool.c
index d241926b4..248145364 100644
--- a/tests/backend/test-threadpool.c
+++ b/tests/backend/test-threadpool.c
@@ -33,9 +33,9 @@ void is_prime(void * arg)
   unsigned long long number = *(unsigned long long *)arg;
   unsigned long long prime = 1;
 
-  for (unsigned long long x = number; x >= sqrt(number); --x)
+  for (unsigned long long x = number-1; x >= sqrt(number); --x)
     {
-      if (number % x == 0)
+      if ((number % x) == 0)
 	{
 	  prime = 0;
 	  break;
@@ -180,7 +180,7 @@ TEST ("process-function")
 {
   // this is what the scheduler would do once it figures out what function a job needs to run
   LumieraThread t;
-  unsigned long long number = 18014398241046527;
+  unsigned long long number = 10000019;
 
   lumiera_threadpool_init();
 
@@ -193,8 +193,9 @@ TEST ("process-function")
 			  &NOBUG_FLAG(NOBUG_ON)); // struct nobug_flag* flag)
 
   // cleanup
-  ECHO("finished waiting");
   lumiera_threadpool_destroy();
+  ECHO("the result is %llu", number);
+
 }
 
 TESTS_END