diff --git a/sigproc/src/Microboone.cxx b/sigproc/src/Microboone.cxx
index 2add2b637..64bb27b23 100644
--- a/sigproc/src/Microboone.cxx
+++ b/sigproc/src/Microboone.cxx
@@ -1124,6 +1124,8 @@ WireCell::Waveform::ChannelMaskMap Microboone::ADCBitShift::apply(int ch, signal
 
     for (int i = 0; i != nbin; i++) {
         int x = signal.at(i);
+        if(x < 0)
+            x = 0;
         s.clear();
         do {
             s.push_back((x & 1));