diff --git a/README.md b/README.md
index 5c30e1e..127844b 100644
--- a/README.md
+++ b/README.md
@@ -123,6 +123,36 @@ If the confgiuration file path isn't specified, the file named `.tengo-tester.ym
 There are some configuration which are file paths.
 If a file path is a relative path, the base directory of the relative path is the directory where the configuration file exists.
 
+## The type of map key should be string in entry.params and test.equal
+
+`entry.params` and `test.equal` is a variable of a Tengo script.
+The type of map key of Tengo script should be string.
+
+https://github.com/d5/tengo/blob/master/docs/tutorial.md#values-and-value-types
+
+So the test fails if the map key whose type isn't string is found.
+
+ex.
+
+```yaml
+---
+entries:
+- name: main
+  script_file: foo.tengo
+  params:
+    foo:
+      true: 1 # the key is boolean
+  tests:
+  - name: test
+    var_name: result
+    equal: hello
+```
+
+```
+$ tengo-tester run
+FATA[0000] format config: convert entry.params: entry_name: main: key: foo: the map key should be string : true
+```
+
 ## LICENSE
 
 [MIT](LICENSE)
diff --git a/pkg/config/config.go b/pkg/config/config.go
index 71bca87..910ccb7 100644
--- a/pkg/config/config.go
+++ b/pkg/config/config.go
@@ -1,5 +1,9 @@
 package config
 
+import (
+	"fmt"
+)
+
 type Config struct {
 	Entries  []Entry
 	LogLevel string `yaml:"log_level"`
@@ -21,3 +25,55 @@ type Test struct {
 	IsNil      bool `yaml:"is_nil"`
 	IsNotNil   bool `yaml:"is_not_nil"`
 }
+
+func ConvertConfig(cfg Config) error {
+	for i, entry := range cfg.Entries {
+		for k, v := range entry.Params {
+			p, err := ConvertMapKey(v)
+			if err != nil {
+				return fmt.Errorf("convert entry.params: entry_name: %s: key: %s: %w", entry.Name, k, err)
+			}
+			entry.Params[k] = p
+		}
+		for j, test := range entry.Tests {
+			p, err := ConvertMapKey(test.Equal)
+			if err != nil {
+				return fmt.Errorf("convert test.Equal: entry_name: %s: test_name: %s: %w", entry.Name, test.Name, err)
+			}
+			test.Equal = p
+			entry.Tests[j] = test
+		}
+		cfg.Entries[i] = entry
+	}
+	return nil
+}
+
+func ConvertMapKey(data interface{}) (interface{}, error) {
+	switch t := data.(type) {
+	case map[interface{}]interface{}:
+		m := make(map[string]interface{}, len(t))
+		for k, v := range t {
+			s, ok := k.(string)
+			if !ok {
+				return nil, fmt.Errorf("the map key should be string: %+v", k)
+			}
+			val, err := ConvertMapKey(v)
+			if err != nil {
+				return nil, fmt.Errorf("key: %s: %w", s, err)
+			}
+			m[s] = val
+		}
+		return m, nil
+	case []interface{}:
+		for i, v := range t {
+			val, err := ConvertMapKey(v)
+			if err != nil {
+				return nil, fmt.Errorf("index: %d: %w", i, err)
+			}
+			t[i] = val
+		}
+		return t, nil
+	default:
+		return data, nil
+	}
+}
diff --git a/pkg/controller/controller.go b/pkg/controller/controller.go
index e63cacb..0b783eb 100644
--- a/pkg/controller/controller.go
+++ b/pkg/controller/controller.go
@@ -147,6 +147,9 @@ func (ctrl Controller) Run(ctx context.Context, wd string) error {
 	if err != nil {
 		return err
 	}
+	if err := config.ConvertConfig(ctrl.Config); err != nil {
+		return fmt.Errorf("format config: %w", err)
+	}
 	for _, entry := range ctrl.Config.Entries {
 		err := ctrl.testEntry(wd, entry, testErrorTpl)
 		if err != nil {