diff --git a/ext/iconv/iconv.c b/ext/iconv/iconv.c
index 157ee41a11a05..d2ddd3e3a6615 100644
--- a/ext/iconv/iconv.c
+++ b/ext/iconv/iconv.c
@@ -43,7 +43,7 @@
 #undef iconv
 #endif
 
-#if defined(__NetBSD__)
+#if defined(__NetBSD__) || defined (__illumos__)
 // unfortunately, netbsd has still the old non posix conformant signature
 // libiconv tends to match the eventual system's iconv too.
 #define ICONV_CONST const