diff --git a/src/basic.rs b/src/basic.rs
index cd22d6f..0899fa9 100644
--- a/src/basic.rs
+++ b/src/basic.rs
@@ -161,7 +161,7 @@ pub fn encode_real8(fnum: f64) -> u64 {
     let ieee = fnum.to_bits();
     let sign = ieee & 0x8000_0000_0000_0000;
     let ieee_exp = (ieee >> 52) as i32 & 0x7ff;
-    let ieee_mant = ieee & 0xf_ffff_ffff_ffff;
+    let ieee_mant = ieee & 0x000f_ffff_ffff_ffff;
 
     let subnorm = (ieee_exp == 0) & (ieee_mant != 0);
     if (ieee_exp == 0) & (ieee_mant == 0) {