diff --git a/helper/plugin/grpc_provisioner.go b/helper/plugin/grpc_provisioner.go
index 14494e462..088e94e4a 100644
--- a/helper/plugin/grpc_provisioner.go
+++ b/helper/plugin/grpc_provisioner.go
@@ -2,6 +2,8 @@ package plugin
 
 import (
 	"log"
+	"strings"
+	"unicode/utf8"
 
 	"github.com/hashicorp/terraform/helper/schema"
 	proto "github.com/hashicorp/terraform/internal/tfplugin5"
@@ -90,7 +92,7 @@ type uiOutput struct {
 
 func (o uiOutput) Output(s string) {
 	err := o.srv.Send(&proto.ProvisionResource_Response{
-		Output: s,
+		Output: toValidUTF8(s, string(utf8.RuneError)),
 	})
 	if err != nil {
 		log.Printf("[ERROR] %s", err)
@@ -145,3 +147,55 @@ func (s *GRPCProvisionerServer) Stop(_ context.Context, req *proto.Stop_Request)
 
 	return resp, nil
 }
+
+// FIXME: backported from go1.13 strings package, remove once terraform is
+//        using go >= 1.13
+// ToValidUTF8 returns a copy of the string s with each run of invalid UTF-8 byte sequences
+// replaced by the replacement string, which may be empty.
+func toValidUTF8(s, replacement string) string {
+	var b strings.Builder
+
+	for i, c := range s {
+		if c != utf8.RuneError {
+			continue
+		}
+
+		_, wid := utf8.DecodeRuneInString(s[i:])
+		if wid == 1 {
+			b.Grow(len(s) + len(replacement))
+			b.WriteString(s[:i])
+			s = s[i:]
+			break
+		}
+	}
+
+	// Fast path for unchanged input
+	if b.Cap() == 0 { // didn't call b.Grow above
+		return s
+	}
+
+	invalid := false // previous byte was from an invalid UTF-8 sequence
+	for i := 0; i < len(s); {
+		c := s[i]
+		if c < utf8.RuneSelf {
+			i++
+			invalid = false
+			b.WriteByte(c)
+			continue
+		}
+		_, wid := utf8.DecodeRuneInString(s[i:])
+		if wid == 1 {
+			i++
+			if !invalid {
+				invalid = true
+				b.WriteString(replacement)
+			}
+			continue
+		}
+		invalid = false
+		b.WriteString(s[i : i+wid])
+		i += wid
+	}
+
+	return b.String()
+}
diff --git a/helper/plugin/grpc_provisioner_test.go b/helper/plugin/grpc_provisioner_test.go
index c64045ab4..9b38daf4a 100644
--- a/helper/plugin/grpc_provisioner_test.go
+++ b/helper/plugin/grpc_provisioner_test.go
@@ -1,5 +1,82 @@
 package plugin
 
-import proto "github.com/hashicorp/terraform/internal/tfplugin5"
+import (
+	"testing"
+	"unicode/utf8"
+
+	"github.com/golang/mock/gomock"
+	"github.com/hashicorp/terraform/helper/schema"
+	proto "github.com/hashicorp/terraform/internal/tfplugin5"
+	mockproto "github.com/hashicorp/terraform/plugin/mock_proto"
+	"github.com/hashicorp/terraform/terraform"
+	context "golang.org/x/net/context"
+)
 
 var _ proto.ProvisionerServer = (*GRPCProvisionerServer)(nil)
+
+type validUTF8Matcher string
+
+func (m validUTF8Matcher) Matches(x interface{}) bool {
+	resp := x.(*proto.ProvisionResource_Response)
+	return utf8.Valid([]byte(resp.Output))
+}
+
+func (m validUTF8Matcher) String() string {
+	return string(m)
+}
+
+func mockProvisionerServer(t *testing.T, c *gomock.Controller) *mockproto.MockProvisioner_ProvisionResourceServer {
+	server := mockproto.NewMockProvisioner_ProvisionResourceServer(c)
+
+	server.EXPECT().Send(
+		validUTF8Matcher("check for valid utf8"),
+	).Return(nil)
+
+	return server
+}
+
+// ensure that a provsioner cannot return invalid utf8 which isn't allowed in
+// the grpc protocol.
+func TestProvisionerInvalidUTF8(t *testing.T) {
+	p := &schema.Provisioner{
+		ConnSchema: map[string]*schema.Schema{
+			"foo": {
+				Type:     schema.TypeString,
+				Optional: true,
+			},
+		},
+
+		Schema: map[string]*schema.Schema{
+			"foo": {
+				Type:     schema.TypeInt,
+				Optional: true,
+			},
+		},
+
+		ApplyFunc: func(ctx context.Context) error {
+			out := ctx.Value(schema.ProvOutputKey).(terraform.UIOutput)
+			out.Output("invalid \xc3\x28\n")
+			return nil
+		},
+	}
+
+	ctrl := gomock.NewController(t)
+	defer ctrl.Finish()
+
+	srv := mockProvisionerServer(t, ctrl)
+	cfg := &proto.DynamicValue{
+		Msgpack: []byte("\x81\xa3foo\x01"),
+	}
+	conn := &proto.DynamicValue{
+		Msgpack: []byte("\x81\xa3foo\xa4host"),
+	}
+	provisionerServer := NewGRPCProvisionerServerShim(p)
+	req := &proto.ProvisionResource_Request{
+		Config:     cfg,
+		Connection: conn,
+	}
+
+	if err := provisionerServer.ProvisionResource(req, srv); err != nil {
+		t.Fatal(err)
+	}
+}