diff --git a/include/fast_float/float_common.h b/include/fast_float/float_common.h
index 2b322d3..f15860b 100644
--- a/include/fast_float/float_common.h
+++ b/include/fast_float/float_common.h
@@ -450,7 +450,7 @@ typedef int64_t am_pow_t; // can't be int_fast16_t because invalid_am_bias
                           // hacks. Needs rewriting this.
 
 // Bias so we can get the real exponent with an invalid adjusted_mantissa.
-constexpr static am_pow_t invalid_am_bias = -0x8000;
+constexpr static am_pow_t invalid_am_bias = -0x800000000000000;
 
 struct adjusted_mantissa {
   am_mant_t mantissa;