diff --git a/test/functional/feature_assumevalid.py b/test/functional/feature_assumevalid.py
index 812315d15bf1f..0936b71ab8a56 100755
--- a/test/functional/feature_assumevalid.py
+++ b/test/functional/feature_assumevalid.py
@@ -88,7 +88,7 @@ def assert_blockchain_height(self, node, height):
                 last_height = current_height
                 if timeout < 0:
                     assert False, "blockchain too short after timeout: %d" % current_height
-                timeout - 0.25
+                timeout -= 0.25
                 continue
             elif current_height > height:
                 assert False, "blockchain too long: %d" % current_height