diff --git a/assert/assert_ext_test.go b/assert/assert_ext_test.go
index 88b0421..2ef1725 100644
--- a/assert/assert_ext_test.go
+++ b/assert/assert_ext_test.go
@@ -1,7 +1,11 @@
 package assert_test
 
 import (
-	"fmt"
+	"go/parser"
+	"go/token"
+	"io/ioutil"
+	"runtime"
+	"strings"
 	"testing"
 
 	"gotest.tools/v3/assert"
@@ -9,7 +13,7 @@ import (
 )
 
 func TestEqual_WithGoldenUpdate(t *testing.T) {
-	t.Run("assert failed with update=false", func(t *testing.T) {
+	t.Run("assert failed with -update=false", func(t *testing.T) {
 		ft := &fakeTestingT{}
 		actual := `not this value`
 		assert.Equal(ft, actual, expectedOne)
@@ -18,23 +22,25 @@ func TestEqual_WithGoldenUpdate(t *testing.T) {
 
 	t.Run("value is updated when -update=true", func(t *testing.T) {
 		patchUpdate(t)
-		ft := &fakeTestingT{}
+		t.Cleanup(func() {
+			resetVariable(t, "expectedOne", "")
+		})
 
 		actual := `this is the
 actual value
-that we are testing against`
-		assert.Equal(ft, actual, expectedOne)
+that we are testing
+`
+		assert.Equal(t, actual, expectedOne)
+
+		raw, err := ioutil.ReadFile(fileName(t))
+		assert.NilError(t, err)
 
-		// reset
-		fmt.Println("WHHHHHHHHHHY")
-		assert.Equal(ft, "\n\n\n", expectedOne)
+		expected := "var expectedOne = `this is the\nactual value\nthat we are testing\n`"
+		assert.Assert(t, strings.Contains(string(raw), expected), "actual=%v", string(raw))
 	})
 }
 
-var expectedOne = `
-
-
-`
+var expectedOne = ``
 
 func patchUpdate(t *testing.T) {
 	source.Update = true
@@ -43,6 +49,26 @@ func patchUpdate(t *testing.T) {
 	})
 }
 
+func fileName(t *testing.T) string {
+	t.Helper()
+	_, filename, _, ok := runtime.Caller(1)
+	assert.Assert(t, ok, "failed to get call stack")
+	return filename
+}
+
+func resetVariable(t *testing.T, varName string, value string) {
+	t.Helper()
+	_, filename, _, ok := runtime.Caller(1)
+	assert.Assert(t, ok, "failed to get call stack")
+
+	fileset := token.NewFileSet()
+	astFile, err := parser.ParseFile(fileset, filename, nil, parser.AllErrors|parser.ParseComments)
+	assert.NilError(t, err)
+
+	err = source.UpdateVariable(filename, fileset, astFile, varName, value)
+	assert.NilError(t, err, "failed to reset file")
+}
+
 type fakeTestingT struct {
 	failNowed bool
 	failed    bool
diff --git a/internal/source/update.go b/internal/source/update.go
index 1b669f6..02e2347 100644
--- a/internal/source/update.go
+++ b/internal/source/update.go
@@ -68,6 +68,23 @@ func UpdateExpectedValue(stackIndex int, x, y interface{}) error {
 		value = y
 	}
 
+	strValue, ok := value.(string)
+	if !ok {
+		debug("value must be type string, got %T", value)
+		return ErrNotFound
+	}
+	return UpdateVariable(filename, fileset, astFile, varName, strValue)
+}
+
+// UpdateVariable writes to filename the contents of astFile with the value of
+// the variable updated to value.
+func UpdateVariable(
+	filename string,
+	fileset *token.FileSet,
+	astFile *ast.File,
+	varName string,
+	value string,
+) error {
 	obj := astFile.Scope.Objects[varName]
 	if obj == nil {
 		return ErrNotFound
@@ -89,9 +106,8 @@ func UpdateExpectedValue(stackIndex int, x, y interface{}) error {
 
 	// TODO: allow a function to wrap the string literal
 	spec.Values[0] = &ast.BasicLit{
-		Kind: token.STRING,
-		// TODO: safer
-		Value: "`" + value.(string) + "`",
+		Kind:  token.STRING,
+		Value: "`" + value + "`",
 	}
 
 	debug("after modification: %v", debugFormatNode{astFile})