diff --git a/third_party/subzero/src/IceBitVector.h b/third_party/subzero/src/IceBitVector.h
index f81810f..742d2b4 100644
--- a/third_party/subzero/src/IceBitVector.h
+++ b/third_party/subzero/src/IceBitVector.h
@@ -768,8 +768,10 @@
     Capacity = std::max(NumBitWords(NewSize), Capacity * 2);
     assert(Capacity > 0 && "realloc-ing zero space");
     auto *NewBits = Alloc.allocate(Capacity);
-    std::memcpy(Bits, NewBits, OldCapacity * sizeof(BitWord));
-    Alloc.deallocate(Bits, OldCapacity);
+    if (Bits) {
+      std::memcpy(NewBits, Bits, OldCapacity * sizeof(BitWord));
+      Alloc.deallocate(Bits, OldCapacity);
+    }
     Bits = NewBits;
 
     clear_unused_bits();