diff --git a/src/dumm/ext/dumm.c b/src/dumm/ext/dumm.c
index cca9a0d0e2..603fac0881 100644
--- a/src/dumm/ext/dumm.c
+++ b/src/dumm/ext/dumm.c
@@ -30,6 +30,8 @@
 #undef PACKAGE_STRING
 #undef PACKAGE_BUGREPORT
 #undef PACKAGE_URL
+/* avoid redefintiion of snprintf etc. */
+#define RUBY_DONT_SUBST
 #include <ruby.h>
 
 static dumm_t *dumm;