diff --git a/setup.cfg b/setup.cfg
index 3def0b1362861f42286d6952d750d142e98488ce..25b7592ad8756325b6ef578645b0e84a04f5dbd8 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -38,7 +38,7 @@ install_requires =
     # incompatible with Tahoe-LAFS'.  So duplicate them here (the ones that
     # have been observed to cause problems).
     Twisted[tls,conch]>=18.4.0
-    tahoe-lafs >=1.14, <1.17
+    tahoe-lafs >=1.14, <1.17, !=1.15
     treq
     pyutil