diff --git a/qupulse/hardware/util.py b/qupulse/hardware/util.py
index ec1a91ea..35dcd01d 100644
--- a/qupulse/hardware/util.py
+++ b/qupulse/hardware/util.py
@@ -250,7 +250,7 @@ def check_invalid_values(ch_data):
     if ch2 is None:
         ch2 = np.zeros(size)
     else:
-        check_invalid_values(ch1)
+        check_invalid_values(ch2)
     marker_data = np.zeros(size, dtype=np.uint16)
     for idx, marker in enumerate(markers):
         if marker is not None:
diff --git a/tests/hardware/util_tests.py b/tests/hardware/util_tests.py
index 9b983c28..7693ca3c 100644
--- a/tests/hardware/util_tests.py
+++ b/tests/hardware/util_tests.py
@@ -98,6 +98,8 @@ def test_size_exception(self):
     def test_range_exception(self):
         with self.assertRaisesRegex(ValueError, "invalid"):
             zhinst_voltage_to_uint16(2.*np.ones(192), None, (None, None, None, None))
+        with self.assertRaisesRegex(ValueError, "invalid"):
+            zhinst_voltage_to_uint16(None, 2.*np.ones(192), (None, None, None, None))
         # this should work
         zhinst_voltage_to_uint16(None, None, (2. * np.ones(192), None, None, None))