diff --git a/artiq/compiler/iodelay.py b/artiq/compiler/iodelay.py
index 47e04472a..099f0953a 100644
--- a/artiq/compiler/iodelay.py
+++ b/artiq/compiler/iodelay.py
@@ -106,6 +106,12 @@ class MUToS(Conv):
     def eval(self, env):
         return self.operand.eval(env) * self.ref_period
 
+    def fold(self, vars=None):
+        if isinstance(self.operand, Const):
+            return Const(self.operand.value * self.ref_period)
+        else:
+            return super().fold(vars)
+
 class SToMU(Conv):
     def __str__(self):
         return "s->mu({})".format(self.operand)
@@ -113,6 +119,12 @@ class SToMU(Conv):
     def eval(self, env):
         return self.operand.eval(env) / self.ref_period
 
+    def fold(self, vars=None):
+        if isinstance(self.operand, Const):
+            return Const(self.operand.value / self.ref_period)
+        else:
+            return super().fold(vars)
+
 class BinOp(Expr):
     def __init__(self, lhs, rhs):
         self.lhs, self.rhs = lhs, rhs