diff --git a/balancer/internal/wrr/random.go b/balancer/internal/wrr/random.go
index fe345f78..ec724f2e 100644
--- a/balancer/internal/wrr/random.go
+++ b/balancer/internal/wrr/random.go
@@ -36,12 +36,14 @@ func NewRandom() WRR {
 	return &randomWRR{}
 }
 
+var grpcrandInt63n = grpcrand.Int63n
+
 func (rw *randomWRR) Next() (item interface{}) {
 	if rw.sumOfWeights == 0 {
 		return nil
 	}
 	// Random number in [0, sum).
-	randomWeight := grpcrand.Int63n(rw.sumOfWeights)
+	randomWeight := grpcrandInt63n(rw.sumOfWeights)
 	for _, item := range rw.items {
 		randomWeight = randomWeight - item.Weight
 		if randomWeight < 0 {
diff --git a/balancer/internal/wrr/wrr_test.go b/balancer/internal/wrr/wrr_test.go
index 03c2b900..2e32b9b6 100644
--- a/balancer/internal/wrr/wrr_test.go
+++ b/balancer/internal/wrr/wrr_test.go
@@ -20,6 +20,7 @@ package wrr
 import (
 	"errors"
 	"math"
+	"math/rand"
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
@@ -97,3 +98,8 @@ func testWRRNext(t *testing.T, newWRR func() WRR) {
 func TestRandomWRRNext(t *testing.T) {
 	testWRRNext(t, NewRandom)
 }
+
+func init() {
+	r := rand.New(rand.NewSource(0))
+	grpcrandInt63n = r.Int63n
+}
diff --git a/vet.sh b/vet.sh
index 7e6ce5eb..11037b94 100755
--- a/vet.sh
+++ b/vet.sh
@@ -75,7 +75,7 @@ git ls-files "*.go" | xargs grep -L "\(Copyright [0-9]\{4,\} gRPC authors\)\|DO
 
 # - Do not import math/rand for real library code.  Use internal/grpcrand for
 #   thread safety.
-git ls-files "*.go" | xargs grep -l '"math/rand"' 2>&1 | (! grep -v '^examples\|^stress\|grpcrand')
+git ls-files "*.go" | xargs grep -l '"math/rand"' 2>&1 | (! grep -v '^examples\|^stress\|grpcrand\|wrr_test')
 
 # - Ensure all ptypes proto packages are renamed when importing.
 git ls-files "*.go" | (! xargs grep "\(import \|^\s*\)\"github.com/golang/protobuf/ptypes/")