diff --git a/devito/passes/clusters/implicit.py b/devito/passes/clusters/implicit.py
index 100ba4842a..60b435dbde 100644
--- a/devito/passes/clusters/implicit.py
+++ b/devito/passes/clusters/implicit.py
@@ -11,6 +11,7 @@
 from devito.symbolics import retrieve_dimensions
 from devito.tools import Bunch, frozendict, timed_pass
 from devito.types import Eq, Symbol
+from devito.types.dimension import BlockDimension
 from devito.types.grid import MultiSubDimension
 
 __all__ = ['generate_implicit']
@@ -226,21 +227,25 @@ def msdim(d):
 
 @singledispatch
 def _lower_msd(dim, cluster):
-    # Retval: (dynamic thickness mapper, iteration dimensions)
+    # Retval: (dynamic thickness mapper, iteration dimension)
     return {}, None
 
 
 @_lower_msd.register(MultiSubDimension)
 def _(dim, cluster):
-    # Pull out the parent MultiSubDimension if blocked etc
+    i_dim = dim.implicit_dimension
+    mapper = {(dim.root, i): dim.functions[i_dim, mM]
+              for i, mM in enumerate(dim.bounds_indices)}
+    return mapper, i_dim
+
+
+@_lower_msd.register(BlockDimension)
+def _(dim, cluster):
+    # Pull out the parent MultiSubDimension
     msd = [d for d in dim._defines if d.is_MultiSub]
     assert len(msd) == 1  # Sanity check. MultiSubDimensions shouldn't be nested.
     msd = msd.pop()
-
-    i_dim = msd.implicit_dimension
-    mapper = {(dim.root, i): msd.functions[i_dim, mM]
-              for i, mM in enumerate(msd.bounds_indices)}
-    return mapper, i_dim
+    return _lower_msd(msd, cluster)
 
 
 def lower_msd(msdims, cluster):