diff --git a/src/dwm1c.rs b/src/dwm1c.rs
index 660f170..2eb532c 100644
--- a/src/dwm1c.rs
+++ b/src/dwm1c.rs
@@ -7,7 +7,8 @@ use crate::{
 // This is an implementation of the approximation of the secondary
 // branch of the Lambert W function
 // with 50 bits of accuracy from Fukushima's paper.
-// It returns f64::NAN if the input is negative, NAN, or larger than or equal to 0.
+// It returns f64::NAN if the `zc` input is negative,
+// or if the `z` input is NAN, or larger than or equal to 0.
 // It returns f64::INFINITY if the input is positive infinity.
 
 /// zc = z + 1/e