diff --git a/lib/spack/spack/solver/asp.py b/lib/spack/spack/solver/asp.py
index ce0ae2ca7f4..af0ddd67ae0 100644
--- a/lib/spack/spack/solver/asp.py
+++ b/lib/spack/spack/solver/asp.py
@@ -1883,7 +1883,11 @@ class Body:
                             continue
                         # skip build dependencies of already-installed specs
                         if concrete_build_deps or dtype != dt.BUILD:
-                            clauses.append(fn.attr("depends_on", spec.name, dep.name, dtype))
+                            clauses.append(
+                                fn.attr(
+                                    "depends_on", spec.name, dep.name, dt.flag_to_string(dtype)
+                                )
+                            )
                             for virtual_name in dspec.virtuals:
                                 clauses.append(
                                     fn.attr("virtual_on_edge", spec.name, dep.name, virtual_name)