diff --git a/lib/Tools/LinearLayout.cpp b/lib/Tools/LinearLayout.cpp
index eddfec16a87b..1f28cbd5ec91 100644
--- a/lib/Tools/LinearLayout.cpp
+++ b/lib/Tools/LinearLayout.cpp
@@ -121,6 +121,11 @@ getInjectiveMat(const LinearLayout &layout) {
 // outDim as columns.  In other words, finds the number of linearly-independent
 // bases for this output dimension.
 int getMatrixRank(std::unique_ptr<uint64_t[]> m, int numRows, int numCols) {
+  // f2reduce underflows if the number of cols is 0, return the rank early in
+  // this case.
+  if (numCols == 0) {
+    return 0;
+  }
   // stride is specified in number of 64-bit words per row, and we pack our
   // matrix so that there's only one uint64_t per row.
   assert(numCols <= 64);