frequency variable sine generator - odd blips when interpolating frequencies

I made a small sine generator:

When I adjust the frequency (I use a MIDI controller, but implemented here with a slider gain for compatibility), I get very odd blips and squeals on the sine output. As if I was generating way too high frequencies for a short while. When I'm done fiddling with the slider (or MIDI controller) the new frequency is set and stable.

Inside the smoothGain Block I have this code to avoid parameter "jumps" (and thus crackling on the output). I's an interpolation between two gain values from start to end of a frame:

function output = smoothGain(input, gain)

% create variables needed
persistent lastGain;

if isempty(lastGain)
    lastGain = 0;
end

% check whether gain has changed
if gain ~= lastGain
    % gain has changed and will be interpolated
    output = input .* repmat(linspace(lastGain, gain, size(input,1))', 1,  size(input,2));
    lastGain = gain;
else
    % gain is the same as before
    output = input .*gain;
end

I have verified that the code works as I expect it in Matlab. But I get the artifacts mentioned before.

Am I missing anything?