diff --git a/environment.fixedversions.yaml b/environment.fixedversions.yaml
index 69077dfd..860039ac 100644
--- a/environment.fixedversions.yaml
+++ b/environment.fixedversions.yaml
@@ -4,6 +4,7 @@ channels:
   - conda-forge
   - defaults
 dependencies:
+  - ipopt
   - snakemake=5.4.0=0
   - snakemake-minimal=5.4.0=py_0
   - affine=2.2.1=py_0