diff --git a/artiq/gateware/dsp/fir.py b/artiq/gateware/dsp/fir.py
index 2753b81bc..d6a4b1ff2 100644
--- a/artiq/gateware/dsp/fir.py
+++ b/artiq/gateware/dsp/fir.py
@@ -156,7 +156,7 @@ class ParallelHBFUpsampler(Module):
         i = [self.i]
         for coeff in coefficients:
             self.parallelism *= 2
-            hbf = ParallelFIR(coeff, self.parallelism, width, **kwargs)
+            hbf = ParallelFIR(coeff, self.parallelism, width + 1, **kwargs)
             self.submodules += hbf
             self.comb += [a.eq(b) for a, b in zip(hbf.i[::2], i)]
             i = hbf.o