diff --git a/libs/ardour/interpolation.cc b/libs/ardour/interpolation.cc
index a4a4b08872..444183a2b1 100644
--- a/libs/ardour/interpolation.cc
+++ b/libs/ardour/interpolation.cc
@@ -64,6 +64,7 @@ Interpolation::interpolate (int channel, nframes_t nframes, Sample *input, Sampl
 	data[channel].data_in       = input;
 	data[channel].data_out      = output;
 	
+	data[channel].input_frames = nframes * _speed;
 	data[channel].output_frames = nframes;
 	data[channel].src_ratio     = 1.0/_speed;