diff --git a/.circleci/config.yml b/.circleci/config.yml
index 269085f..2029eda 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -4,7 +4,7 @@ jobs:
   build:
     working_directory: /go/src/github.com/qri-io/starlib
     docker:
-      - image: circleci/golang:1.12
+      - image: circleci/golang:latest
     environment:
       TEST_RESULTS: /tmp/test-results
       GO111MODULE: "on"
diff --git a/util/util_test.go b/util/util_test.go
index 76b8a06..8b99c06 100644
--- a/util/util_test.go
+++ b/util/util_test.go
@@ -153,13 +153,19 @@ type invalidCustomType struct {
 
 type customType invalidCustomType
 
-func (t *customType) UnmarshalStarlark(v starlark.Value) error {
+var (
+	_ Unmarshaler    = (*customType)(nil)
+	_ Marshaler      = (*customType)(nil)
+	_ starlark.Value = (*customType)(nil)
+)
+
+func (c *customType) UnmarshalStarlark(v starlark.Value) error {
 	// asserts
 	if v.Type() != "struct" {
 		return fmt.Errorf("not expected top level type, want struct, got %q", v.Type())
 	}
 	if _, ok := v.(*starlarkstruct.Struct).Constructor().(*customType); !ok {
-		return fmt.Errorf("not expected construct type got %T, want %T", v.(*starlarkstruct.Struct).Constructor(), t)
+		return fmt.Errorf("not expected construct type got %T, want %T", v.(*starlarkstruct.Struct).Constructor(), c)
 	}
 
 	// TODO: refactoring transform data
@@ -172,15 +178,15 @@ func (t *customType) UnmarshalStarlark(v starlark.Value) error {
 	data := starlark.StringDict{}
 	v.(*starlarkstruct.Struct).ToStringDict(data)
 
-	*t = customType{
+	*c = customType{
 		Foo: mustInt64(data["foo"]),
 	}
 	return nil
 }
 
-func (t *customType) MarshalStarlark() (starlark.Value, error) {
+func (c *customType) MarshalStarlark() (starlark.Value, error) {
 	v := starlarkstruct.FromStringDict(&customType{}, starlark.StringDict{
-		"foo": starlark.MakeInt64(t.Foo),
+		"foo": starlark.MakeInt64(c.Foo),
 	})
 	return v, nil
 }
@@ -200,7 +206,3 @@ func (c customType) Truth() starlark.Bool {
 func (c customType) Hash() (uint32, error) {
 	return 0, fmt.Errorf("unhashable: %s", c.Type())
 }
-
-var _ Unmarshaler = (*customType)(nil)
-var _ Marshaler = (*customType)(nil)
-var _ starlark.Value = (*customType)(nil)