diff --git a/flags/flags.go b/flags/flags.go
index 637a96d..84880e4 100644
--- a/flags/flags.go
+++ b/flags/flags.go
@@ -145,16 +145,16 @@ func expandOpts(opts any) ([]Flag, error) {
 	for j := 0; j < v.NumField(); j++ {
 		field := v.Field(j)
 		if field.Kind() != reflect.Struct {
-			return nil, ParseError{fmt.Sprintf("needs a struct: %#v", field.Kind())}
+			return nil, ParseError{fmt.Sprintf("field must be a struct: %#v", field.Kind())}
 		}
 		if !field.CanAddr() {
-			return nil, ParseError{fmt.Sprintf("needs an addressable type: %#v", field.Type())}
+			return nil, ParseError{fmt.Sprintf("field must be addressable type: %#v", field.Type())}
 		}
 		// NOTE: Addr() gets the underlying address:
 		if f, ok := field.Addr().Interface().(Flag); ok {
 			flags = append(flags, f)
 		} else {
-			return nil, ParseError{fmt.Sprintf("needs a []Flag interface: %#v", field.Type())}
+			return nil, ParseError{fmt.Sprintf("field must be a []Flag interface: %#v", field.Type())}
 		}
 	}
 	return flags, nil
diff --git a/flags/flags_test.go b/flags/flags_test.go
index 3e7a5b3..84794ad 100644
--- a/flags/flags_test.go
+++ b/flags/flags_test.go
@@ -57,6 +57,35 @@ func TestParseArgsHelp(t *testing.T) {
 	assert.True(t, opts.Help.Val)
 }
 
+func TestExpandOpts(t *testing.T) {
+	opts := initOpts()
+	flags, err := expandOpts(&opts)
+	assert.NoError(t, err)
+	assert.Equal(t, &opts.Descriptor, flags[0])
+	assert.Equal(t, &opts.Tags, flags[1])
+	assert.Equal(t, &opts.Source, flags[2])
+	assert.Equal(t, &opts.Write, flags[3])
+	assert.Equal(t, &opts.Help, flags[4])
+}
+
+func TestExpandOptsNeedsPointer(t *testing.T) {
+	opts := initOpts()
+	_, err := expandOpts(opts)
+	assert.ErrorContains(t, err, "needs a pointer to a struct")
+	err = ParseArgs(helpErr, nil, opts)
+	assert.ErrorContains(t, err, "needs a pointer to a struct")
+}
+
+func TestExpandOptsNeedsStruct(t *testing.T) {
+	badOpts := struct {
+		bar string
+		foo bool
+	}{"bar", false}
+
+	_, err := expandOpts(&badOpts)
+	assert.ErrorContains(t, err, "field must be a struct")
+}
+
 func TestParseArgsString(t *testing.T) {
 	args := []string{"--source", "foo"}
 	opts := initOpts()