diff --git a/src/complex/cacosh.c b/src/complex/cacosh.c
index 76127f75..55b857ce 100644
--- a/src/complex/cacosh.c
+++ b/src/complex/cacosh.c
@@ -1,6 +1,6 @@
 #include "complex_impl.h"
 
-/* acosh(z) = i acos(z) */
+/* acosh(z) = ±i acos(z) */
 
 double complex cacosh(double complex z)
 {