diff --git a/vlib/builtin/cfns.v b/vlib/builtin/cfns.v
index 60d759c6e3..fa685d6891 100644
--- a/vlib/builtin/cfns.v
+++ b/vlib/builtin/cfns.v
@@ -36,7 +36,7 @@ fn C.fputs(byteptr) int
 fn C.fflush(byteptr) int
 // TODO define args in these functions
 fn C.fseek() int
-fn C.fopen() int
+fn C.fopen() voidptr
 fn C.fwrite() int
 fn C.fclose() int
 fn C.pclose() int