diff --git a/vips/conversion.go b/vips/conversion.go
index 5e7c09e9..072623c0 100644
--- a/vips/conversion.go
+++ b/vips/conversion.go
@@ -1,5 +1,6 @@
 package vips
 
+// #cgo CFLAGS: -std=c99
 // #include "conversion.h"
 import "C"