diff --git a/tools/run_tests/build_ruby.sh b/tools/run_tests/build_ruby.sh
index de96413bc12..b6c4e32b7ec 100755
--- a/tools/run_tests/build_ruby.sh
+++ b/tools/run_tests/build_ruby.sh
@@ -31,7 +31,7 @@
 
 set -ex
 
-export CONFIG=${CONFIG:-opt}
+export GRPC_CONFIG=${CONFIG:-opt}
 
 # change to grpc's ruby directory
 cd $(dirname $0)/../../src/ruby