diff --git a/transport/grpc/balancer.go b/transport/grpc/balancer.go
index 5731bb9ea..0181792df 100644
--- a/transport/grpc/balancer.go
+++ b/transport/grpc/balancer.go
@@ -49,6 +49,11 @@ type Builder struct {
 
 // Build creates a grpc Picker.
 func (b *Builder) Build(info base.PickerBuildInfo) gBalancer.Picker {
+	if len(info.ReadySCs) == 0 {
+		// Block the RPC until a new picker is available via UpdateState().
+		return base.NewErrPicker(gBalancer.ErrNoSubConnAvailable)
+	}
+
 	nodes := make([]selector.Node, 0)
 	for conn, info := range info.ReadySCs {
 		ins, _ := info.Address.Attributes.Value("rawServiceInstance").(*registry.ServiceInstance)