diff --git a/dedal/spack_factory/SpackOperationCreateCache.py b/dedal/spack_factory/SpackOperationCreateCache.py
index 41cfc8454136fd0096f6dcf78c0df62de004c6e6..8d6125fb3ebc3d505509ac29841b0975a0b8e768 100644
--- a/dedal/spack_factory/SpackOperationCreateCache.py
+++ b/dedal/spack_factory/SpackOperationCreateCache.py
@@ -42,7 +42,10 @@ class SpackOperationCreateCache(SpackOperation):
         if self.spack_config.gpg:
             signed = True
             self.create_gpg_keys()
-        self.add_mirror('local_cache', self.spack_config.buildcache_dir, signed=signed, autopush=signed,
+        self.add_mirror('local_cache',
+                        str(self.spack_config.buildcache_dir),
+                        signed=signed,
+                        autopush=signed,
                         global_mirror=False)
         self.logger.info(f'Added mirror for {self.spack_config.env.name}')
         super().install_packages(jobs=jobs, signed=signed, debug=debug, fresh=True)