diff --git a/test/helper/testgem/ext/extconf.rb b/test/helper/testgem/ext/extconf.rb
index 0f5c2c2fc..caaf5c703 100644
--- a/test/helper/testgem/ext/extconf.rb
+++ b/test/helper/testgem/ext/extconf.rb
@@ -3,7 +3,7 @@
 require 'mkmf'
 require 'rbconfig'
 
-$CFLAGS << " -Wall -Wcast-qual -Wwrite-strings -Wconversion -Wmissing-noreturn -Winline"
+$CFLAGS << " -std=c99 -Wall -Wcast-qual -Wwrite-strings -Wconversion -Wmissing-noreturn -Winline"
 
 have_header('sys/types.h') || raise("have_header should find std header files")
 have_func('rb_thread_call_with_gvl') || raise("have_func should find libruby symbols")