diff --git a/artiq/devices/pdq2/driver.py b/artiq/devices/pdq2/driver.py
index 851adb5c8..aa8b8ae31 100644
--- a/artiq/devices/pdq2/driver.py
+++ b/artiq/devices/pdq2/driver.py
@@ -6,7 +6,7 @@ import struct
 
 import serial
 
-from artiq.wavesynth.interpolate import discrete_compensate
+from artiq.wavesynth.coefficients import discrete_compensate
 
 
 logger = logging.getLogger(__name__)
diff --git a/artiq/wavesynth/interpolate.py b/artiq/wavesynth/coefficients.py
similarity index 90%
rename from artiq/wavesynth/interpolate.py
rename to artiq/wavesynth/coefficients.py
index 8e133c1c7..c19c4f7a5 100644
--- a/artiq/wavesynth/interpolate.py
+++ b/artiq/wavesynth/coefficients.py
@@ -24,11 +24,14 @@ def _interpolate(time, data, sample_times, order=3):
 
 
 def discrete_compensate(c):
-    if len(c) > 2:
-        c[1] += c[2]/2
-    if len(c) > 3:
-        c[1] += c[3]/6
+    l = len(c)
+    if l > 2:
+        c[1] += c[2]/2.
+    if l > 3:
+        c[1] += c[3]/6.
         c[2] += c[3]
+    if l > 4:
+        raise ValueError("only third-order splines supported")
 
 
 def _zip_program(times, channels, target):
diff --git a/artiq/wavesynth/compute_samples.py b/artiq/wavesynth/compute_samples.py
index 76841c35e..c615c9dd1 100644
--- a/artiq/wavesynth/compute_samples.py
+++ b/artiq/wavesynth/compute_samples.py
@@ -1,7 +1,7 @@
 from copy import copy
 from math import cos, pi
 
-from artiq.wavesynth.interpolate import discrete_compensate
+from artiq.wavesynth.coefficients import discrete_compensate
 
 
 class Spline: