diff --git a/include/pangolin/utils/picojson.h b/include/pangolin/utils/picojson.h
index 3791ee6b7..85252691f 100644
--- a/include/pangolin/utils/picojson.h
+++ b/include/pangolin/utils/picojson.h
@@ -69,7 +69,9 @@ extern "C" {
 
 // experimental support for int64_t (see README.mkdn for detail)
 #ifdef PICOJSON_USE_INT64
-# define __STDC_FORMAT_MACROS
+# ifndef __STDC_FORMAT_MACROS
+#  define __STDC_FORMAT_MACROS
+# endif
 # include <errno.h>
 # include <inttypes.h>
 #endif // PICOJSON_USE_INT64