diff --git a/environment.yml b/environment.yml
index 4b9343130..c45d8102a 100644
--- a/environment.yml
+++ b/environment.yml
@@ -54,3 +54,6 @@ dependencies:
   - threadpoolctl
   - pip:
     - git+https://github.com/OpenFreeEnergy/gufe@main
+  - run_constrained:
+    # drop this pin when handled upstream in espaloma-feedstock
+    - smirnoff99frosst>=1.1.0.1  #https://github.com/openforcefield/smirnoff99Frosst/issues/109