diff --git a/devito/propagator.py b/devito/propagator.py
index 3c325608cd..d7d7acdda9 100644
--- a/devito/propagator.py
+++ b/devito/propagator.py
@@ -6,6 +6,7 @@
 import numpy as np
 from sympy import Indexed, IndexedBase, symbols
 from sympy.abc import t, x, y, z
+from sympy.utilities.iterables import postorder_traversal
 
 import cgen_wrapper as cgen
 from codeprinter import ccode
@@ -560,22 +561,22 @@ def time_substitutions(self, sympy_expr):
         :param sympy_expr: The Sympy expression to process
         :returns: The expression after the substitutions
         """
-        if isinstance(sympy_expr, Indexed):
-            array_term = sympy_expr
+        subs_dict = {}
 
-            if not str(array_term.base.label) in self.save_vars:
-                raise ValueError("Invalid variable '%s' in sympy expression. Did you add it to the operator's params?" %
-                                 str(array_term.base.label))
+        for arg in postorder_traversal(sympy_expr):
+            if isinstance(arg, Indexed):
+                array_term = arg
 
-            if not self.save_vars[str(array_term.base.label)]:
-                array_term = array_term.xreplace(self.t_replace)
+                if not str(array_term.base.label) in self.save_vars:
+                    raise ValueError(
+                        "Invalid variable '%s' in sympy expression. Did you add it to the operator's params?"
+                        % str(array_term.base.label)
+                    )
 
-            return array_term
-        else:
-            for arg in sympy_expr.args:
-                sympy_expr = sympy_expr.subs(arg, self.time_substitutions(arg))
+                if not self.save_vars[str(array_term.base.label)]:
+                    subs_dict[arg] = array_term.xreplace(self.t_replace)
 
-        return sympy_expr
+        return sympy_expr.xreplace(subs_dict)
 
     def add_time_loop_stencil(self, stencil, before=False):
         """Add a statement either before or after the main spatial loop, but still inside the time loop.