diff --git a/setup.cfg b/setup.cfg
index 8fd1f2967bb0a0c0e92db598646aa4f7cd5d0f94..5f7b828e9c1ff5fec021659b68d5657d350f7466 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -41,7 +41,10 @@ install_requires =
     # incompatible with Tahoe-LAFS'.  So duplicate them here (the ones that
     # have been observed to cause problems).
     Twisted[tls,conch] >= 19.10.0
-    tahoe-lafs >=1.17,<1.18
+
+    # Tahoe has no stable Python API but we use its Python API so there's
+    # basically no wiggle room here.
+    tahoe-lafs == 1.17.0
     treq
     pyutil
     prometheus-client