diff --git a/devito/passes/clusters/implicit.py b/devito/passes/clusters/implicit.py
index 60b435dbde..590b0a0377 100644
--- a/devito/passes/clusters/implicit.py
+++ b/devito/passes/clusters/implicit.py
@@ -172,7 +172,7 @@ def callback(self, clusters, prefix):
                 continue
 
             # Get the dynamic thickness mapper for the given MultiSubDomain
-            mapper, dims = lower_msd({i.dim for i in ispace}, c)
+            mapper, dims = lower_msd(ispace.itdims, c)
             if dims:
                 # An Explicit MSD
                 continue