diff --git a/circuit/codex/sample_cells.circom b/circuit/codex/sample_cells.circom
index cb03e92..6a1d2ff 100644
--- a/circuit/codex/sample_cells.circom
+++ b/circuit/codex/sample_cells.circom
@@ -112,7 +112,7 @@ log("top root check = ", mtop.recRoot == dataSetRoot);
   //
   // then we prove the individual sampled cells
 
-  component lg = Log2(maxDepth);                          // we allow at most 2^32 cells per slot
+  component lg = Log2_CircomWitnessCalc_Hack(maxDepth);           // we allow at most 2^32 cells per slot
   lg.inp <== nCellsPerSlot;
 
   // NOTE: in general we need for the Merkle prover the binary decomposition
diff --git a/circuit/lib/log2.circom b/circuit/lib/log2.circom
index bb5d626..2cfa5c2 100644
--- a/circuit/lib/log2.circom
+++ b/circuit/lib/log2.circom
@@ -36,6 +36,60 @@ template Log2(n) {
     0 === (mask[i] - mask[i+1]) * (i + 1 - out);
   }
 
+  inp === sum;
+}
+
+//------------------------------------------------------------------------------
+//
+// a version of `Log2(n)` which works with `circom-witnesscalc`
+//
+// yeah this is very ugly. Direct your complaints to the iden3 people. ¯\_(ツ)_/¯
+//
+
+template Log2_CircomWitnessCalc_Hack(n) {
+
+  signal input  inp;
+  signal output out;
+  signal output mask[n+1];
+
+  // compute the logarithm
+  // as `circom-witnesscalc` does not support any kind of dynamic computation
+  // and support for functions, and other things also seems to be limited
+  // we have to implement this here as a static computation instead
+  // but even that's not enough for `circom-witnesscalc`! hence this uglyness 
+  var x    = inp;
+  var cnt  = -1;
+  signal done[257]; done[0] <--  0;     // we need *signals* here because `circom-witnesscalc` doesn't work otherwise (seriously...)
+  signal log2[257]; log2[0] <-- -1;     // but hopefully circom will optimize these away...
+  for(var i=0; i<256; i++) {
+    var not_done  = (done[i] == 0);
+    var do_switch = (x==0) && not_done;
+    log2[i+1] <-- not_done  ? cnt : log2[i];
+    done[i+1] <-- do_switch ? 1   : done[i];
+    cnt   = cnt + 1;
+    x     = x >> 1;
+  }
+
+  // mask will be a vector [1,1,1,...1,0,...,0,0]
+  // which can change only where index == out
+
+  for(var i=0; i<=n; i++) {
+    mask[i] <-- ((2**i) < inp) ? 1 : 0;
+  }
+
+  out <-- log2[256];
+
+  log("Log2 hack: input = ",inp," | logarithm = ",out);
+
+  mask[0] === 1;
+  mask[n] === 0;
+
+  var sum = 0;
+  for(var i=0; i<n; i++) {
+    sum += (2**(i+1)) * (mask[i] - mask[i+1]);
+    0 === (mask[i] - mask[i+1]) * (i + 1 - out);
+  }
+
   inp === sum;
 
 }