diff --git a/README.md b/README.md
index d9f581c..ac20185 100644
--- a/README.md
+++ b/README.md
@@ -364,6 +364,21 @@ ex.
 
 We can test external Tengo scripts with [suzuki-shunsuke/tengo-tester](https://github.com/suzuki-shunsuke/tengo-tester).
 
+## Restriction: map key should be string
+
+* meta
+* phase.meta
+* task.meta
+* task.items
+
+NG
+
+```yaml
+---
+meta:
+  true: foo # invalid key: key should be string
+```
+
 ## Configuration Reference
 
 ```yaml
diff --git a/go.mod b/go.mod
index 0c12e22..6ba2315 100644
--- a/go.mod
+++ b/go.mod
@@ -16,6 +16,7 @@ require (
 	github.com/sirupsen/logrus v1.7.0
 	github.com/stretchr/testify v1.6.1
 	github.com/suzuki-shunsuke/go-ci-env v0.2.1
+	github.com/suzuki-shunsuke/go-convmap v0.1.0
 	github.com/suzuki-shunsuke/go-dataeq v1.0.1
 	github.com/suzuki-shunsuke/go-error-with-exit-code v1.0.0
 	github.com/suzuki-shunsuke/go-findconfig v0.1.0
diff --git a/go.sum b/go.sum
index a14bb7d..f8df1ac 100644
--- a/go.sum
+++ b/go.sum
@@ -150,6 +150,8 @@ github.com/stretchr/testify v1.6.1 h1:hDPOHmpOpP40lSULcqw7IrRb/u7w6RpDC9399XyoNd
 github.com/stretchr/testify v1.6.1/go.mod h1:6Fq8oRcR53rry900zMqJjRRixrwX3KX962/h/Wwjteg=
 github.com/suzuki-shunsuke/go-ci-env v0.2.1 h1:BXyeehL7uHtQ8cM+xwLzfzZ8fEhrvcIXJtYe0MEqN0o=
 github.com/suzuki-shunsuke/go-ci-env v0.2.1/go.mod h1:kO9UgcQIAH4Pu4ESkUJHPuQEtesxHPKkpQqeJHJzzdk=
+github.com/suzuki-shunsuke/go-convmap v0.1.0 h1:0qzSC9IAzYdylNJvgzFpxbD/KeWpuVM608d/S0VjYsU=
+github.com/suzuki-shunsuke/go-convmap v0.1.0/go.mod h1:S0yDqBPFU0lmlHhoMKNSSHDRt87tUyI+KY/fBuXNn14=
 github.com/suzuki-shunsuke/go-dataeq v1.0.1 h1:ruo7fZ2tT1g5wSWxNTXbDzbdKZRyycnUiAQrhkrOWxw=
 github.com/suzuki-shunsuke/go-dataeq v1.0.1/go.mod h1:y9Jf/g370ehd4VBPajzO2Ir8kj+Y4OR+yRu+T5CiK88=
 github.com/suzuki-shunsuke/go-error-with-exit-code v1.0.0 h1:oVXrrYNGBq4POyITQNWKzwsYz7B2nUcqtDbeX4BfeEc=
diff --git a/pkg/config/config.go b/pkg/config/config.go
index 51ac720..7ee3e68 100644
--- a/pkg/config/config.go
+++ b/pkg/config/config.go
@@ -1,10 +1,12 @@
 package config
 
 import (
+	"fmt"
 	"os"
 
 	"github.com/suzuki-shunsuke/buildflow/pkg/expr"
 	"github.com/suzuki-shunsuke/go-ci-env/cienv"
+	"github.com/suzuki-shunsuke/go-convmap/convmap"
 )
 
 type Phase struct {
@@ -52,12 +54,29 @@ type Env struct {
 	CI           bool
 }
 
+func convertMeta(meta map[string]interface{}) error {
+	for k, a := range meta {
+		b, err := convmap.Convert(a)
+		if err != nil {
+			return fmt.Errorf("parse meta: %w", err)
+		}
+		meta[k] = b
+	}
+	return nil
+}
+
 func Set(cfg Config) (Config, error) {
 	cfg = setDefault(setEnv(cfg))
+	if err := convertMeta(cfg.Meta); err != nil {
+		return cfg, fmt.Errorf(".meta is invalid: %w", err)
+	}
 	for i, phase := range cfg.Phases {
+		if err := convertMeta(phase.Meta); err != nil {
+			return cfg, fmt.Errorf("phase is invalid: %s: %w", phase.Name, err)
+		}
 		for j, task := range phase.Tasks {
 			if err := task.Set(); err != nil {
-				return cfg, err
+				return cfg, fmt.Errorf("task is invalid: %w", err)
 			}
 			phase.Tasks[j] = task
 		}
diff --git a/pkg/config/task.go b/pkg/config/task.go
index a83a749..4f548f6 100644
--- a/pkg/config/task.go
+++ b/pkg/config/task.go
@@ -6,6 +6,7 @@ import (
 	"github.com/suzuki-shunsuke/buildflow/pkg/constant"
 	"github.com/suzuki-shunsuke/buildflow/pkg/execute"
 	"github.com/suzuki-shunsuke/buildflow/pkg/expr"
+	"github.com/suzuki-shunsuke/go-convmap/convmap"
 )
 
 type Task struct {
@@ -49,6 +50,16 @@ func (task *Task) Set() error {
 		task.CompiledItems = Items{
 			Program: prog,
 		}
+	} else if task.Items != nil {
+		a, err := convmap.Convert(task.Items)
+		if err != nil {
+			return err
+		}
+		task.Items = a
+	}
+
+	if err := convertMeta(task.Meta); err != nil {
+		return err
 	}
 
 	return nil