diff --git a/loopy/symbolic.py b/loopy/symbolic.py
index 400090f067e799ea48b6cde12b691ea919b5407f..9a753a6630590f05bd4377c1e02863eebb0bf834 100644
--- a/loopy/symbolic.py
+++ b/loopy/symbolic.py
@@ -1090,6 +1090,8 @@ def simplify_using_aff(kernel, expr):
 
     domain = kernel.get_inames_domain(inames)
 
+    from pymbolic.mapper.evaluator import UnknownVariableError
+
     try:
         with isl.SuppressedWarnings(kernel.isl_context):
             aff = aff_from_expr(domain.space, expr)
@@ -1097,6 +1099,8 @@ def simplify_using_aff(kernel, expr):
         return expr
     except TypeError:
         return expr
+    except UnknownVariableError:
+        return expr
 
     # FIXME: Deal with assumptions, too.
     aff = aff.gist(domain)