diff --git a/devito/core/autotuning.py b/devito/core/autotuning.py
index cb2209690a..23dac69d61 100644
--- a/devito/core/autotuning.py
+++ b/devito/core/autotuning.py
@@ -1,7 +1,6 @@
 from collections import OrderedDict
 from itertools import combinations, product
 from functools import total_ordering
-from sympy import SympifyError, sympify
 
 from devito.arch import KNL, KNL7210
 from devito.ir import Backward, retrieve_iteration_tree
@@ -9,6 +8,7 @@
 from devito.mpi.distributed import MPI, MPINeighborhood
 from devito.mpi.routines import MPIMsgEnriched
 from devito.parameters import configuration
+from devito.symbolics import normalize_args
 from devito.tools import filter_ordered, flatten, is_integer, prod
 from devito.types import Timer
 
@@ -274,13 +274,7 @@ def calculate_nblocks(tree, blockable):
 
 
 def generate_block_shapes(blockable, args, level):
-    # Make sure all params are substitutable (ie cannot raise SympifyError)
-    rargs = {}
-    for k, v in args.items():
-        try:
-            rargs[k] = sympify(v, strict=True)
-        except SympifyError:
-            continue
+    args = normalize_args(args)
 
     if not blockable:
         raise ValueError
@@ -292,7 +286,7 @@ def generate_block_shapes(blockable, args, level):
     # Generate level-0 block shapes
     level_0 = [d for d, v in mapper.items() if v == 0]
     # Max attemptable block shape
-    max_bs = tuple((d.step, d.symbolic_size.subs(rargs)) for d in level_0)
+    max_bs = tuple((d.step, d.symbolic_size.subs(args)) for d in level_0)
     # Defaults (basic mode)
     ret = [tuple((d.step, v) for d in level_0) for v in options['blocksize-l0']]
     # Always try the entire iteration space (degenerate block)
diff --git a/devito/symbolics/manipulation.py b/devito/symbolics/manipulation.py
index 694f85b69e..eb35c68e91 100644
--- a/devito/symbolics/manipulation.py
+++ b/devito/symbolics/manipulation.py
@@ -2,7 +2,7 @@
 from collections.abc import Iterable
 from functools import singledispatch
 
-from sympy import Pow, Add, Mul, Min, Max
+from sympy import Pow, Add, Mul, Min, Max, SympifyError, sympify
 from sympy.core.add import _addsort
 from sympy.core.mul import _mulsort
 
@@ -16,7 +16,8 @@
 from devito.types.relational import Le, Lt, Gt, Ge
 
 __all__ = ['xreplace_indices', 'pow_to_mul', 'indexify', 'subs_op_args',
-           'uxreplace', 'Uxmapper', 'reuse_if_untouched', 'evalrel']
+           'normalize_args', 'uxreplace', 'Uxmapper', 'reuse_if_untouched',
+           'evalrel']
 
 
 def uxreplace(expr, rule):
@@ -269,6 +270,22 @@ def subs_op_args(expr, args):
     return expr.subs({i.name: args[i.name] for i in expr.free_symbols if i.name in args})
 
 
+def normalize_args(args):
+    """
+    Produce a new `args` dictionary in which only the actually substitutable
+    arguments are retained. This ensures that none of the subsequent substitutions
+    will throw `SympifyError` exceptions.
+    """
+    retval = {}
+    for k, v in args.items():
+        try:
+            retval[k] = sympify(v, strict=True)
+        except SympifyError:
+            continue
+
+    return retval
+
+
 def reuse_if_untouched(expr, args, evaluate=False):
     """
     Reconstruct `expr` iff any of the provided `args` is different than