diff --git a/lib/spack/spack/spec.py b/lib/spack/spack/spec.py
index 2edf381bd96..200c12fae3d 100644
--- a/lib/spack/spack/spec.py
+++ b/lib/spack/spack/spec.py
@@ -3193,6 +3193,7 @@ def _constrain_dependencies(self, other: "Spec") -> bool:
                 depflag=dep_spec_copy.depflag,
                 virtuals=dep_spec_copy.virtuals,
                 direct=dep_spec_copy.direct,
+                when=dep_spec_copy.when,
             )
             changed = True