diff --git a/libc/src/__support/float_to_string.h b/libc/src/__support/float_to_string.h
--- a/libc/src/__support/float_to_string.h
+++ b/libc/src/__support/float_to_string.h
@@ -389,8 +389,8 @@
   cpp::UInt<MID_INT_SIZE + MANT_INT_SIZE> val(large);
   // TODO: Find a better way to force __uint128_t to be UInt<128>
   cpp::UInt<MANT_INT_SIZE> wide_mant(0);
-  wide_mant[0] = static_cast<size_t>(mantissa & (uint64_t(-1)));
-  wide_mant[1] = static_cast<size_t>(mantissa >> 64);
+  wide_mant[0] = static_cast<uint64_t>(mantissa & UINT64_MAX);
+  wide_mant[1] = static_cast<uint64_t>(mantissa >> 64);
   val = (val * wide_mant) >> shift_amount;
 
   return static_cast<uint32_t>(