diff --git a/include/fast_float/bigint.h b/include/fast_float/bigint.h
index cb41d91..2697e7c 100644
--- a/include/fast_float/bigint.h
+++ b/include/fast_float/bigint.h
@@ -32,7 +32,7 @@ typedef span<limb> limb_span;
 // of bits required to store the largest bigint, which is
 // `log2(10**(digits + max_exp))`, or `log2(10**(767 + 342))`, or
 // ~3600 bits, so we round to 4000.
-typedef uint_fast16_t bigint_bits_t;
+typedef int_fast16_t bigint_bits_t;
 constexpr bigint_bits_t bigint_bits = 4000;
 constexpr limb_t bigint_limbs = bigint_bits / limb_bits;