diff --git a/artiq/compiler/transforms/iodelay_estimator.py b/artiq/compiler/transforms/iodelay_estimator.py
index aaae55676..a28e9e793 100644
--- a/artiq/compiler/transforms/iodelay_estimator.py
+++ b/artiq/compiler/transforms/iodelay_estimator.py
@@ -258,7 +258,7 @@ class IODelayEstimator(algorithm.Visitor):
                 args = {}
                 for kw_node in node.keywords:
                     args[kw_node.arg] = kw_node.value
-                for arg_name, arg_node in zip(typ.args, node.args[offset:]):
+                for arg_name, arg_node in zip(list(typ.args)[offset:], node.args):
                     args[arg_name] = arg_node
 
                 free_vars = delay.duration.free_vars()
diff --git a/lit-test/test/iodelay/class.py b/lit-test/test/iodelay/class.py
new file mode 100644
index 000000000..32580c4c6
--- /dev/null
+++ b/lit-test/test/iodelay/class.py
@@ -0,0 +1,12 @@
+# RUN: %python -m artiq.compiler.testbench.signature %s >%t
+# RUN: OutputCheck %s --file-to-check=%t
+
+# CHECK-L: g: ()->NoneType delay(s->mu(1.0) mu)
+def g(i):
+    i.f(1.0)
+
+class c:
+    def f(self, x):
+        delay(x)
+
+g(c())