diff --git a/tests/metrics/test_thread_cpu.py b/tests/metrics/test_thread_cpu.py
index 504fba0211..e88fe3b126 100644
--- a/tests/metrics/test_thread_cpu.py
+++ b/tests/metrics/test_thread_cpu.py
@@ -116,7 +116,6 @@ class ThreadCPUMetricsTestCase(unittest.TestCase):
             secs = after[thread_type] - before.get(thread_type, 0.0)
             return Duration(seconds=secs)
 
-        threads_delta = sum(after.values()) - sum(before.values())
         self.assertGreaterEqual(delta("main"), BURN - TOLERANCE)
         self.assertGreaterEqual(delta("test-thread"), BURN - TOLERANCE)
 
@@ -128,4 +127,5 @@ class ThreadCPUMetricsTestCase(unittest.TestCase):
         tick = 1 / os.sysconf("SC_CLK_TCK")
         slack = (len(os.listdir("/proc/self/task")) + 2) * tick
 
+        threads_delta = sum(after.values()) - sum(before.values())
         self.assertLessEqual(abs(threads_delta - process_delta), slack)