diff --git a/tools/internal_ci/linux/grpc_run_tests_matrix.sh b/tools/internal_ci/linux/grpc_run_tests_matrix.sh
index 4e7515227bd..f9acd814ae8 100755
--- a/tools/internal_ci/linux/grpc_run_tests_matrix.sh
+++ b/tools/internal_ci/linux/grpc_run_tests_matrix.sh
@@ -22,6 +22,7 @@ source tools/internal_ci/helper_scripts/prepare_build_linux_rc
 
 # If this is a PR using RUN_TESTS_FLAGS var, then add flags to filter tests
 if [ -n "$KOKORO_GITHUB_PULL_REQUEST_NUMBER" ] && [ -n "$RUN_TESTS_FLAGS" ]; then
+  sudo apt-get update
   sudo apt-get install -y jq
   ghprbTargetBranch=$(curl -s https://api.github.com/repos/grpc/grpc/pulls/$KOKORO_GITHUB_PULL_REQUEST_NUMBER | jq -r .base.ref)
   export RUN_TESTS_FLAGS="$RUN_TESTS_FLAGS --filter_pr_tests --base_branch origin/$ghprbTargetBranch"