@@ -1,9 +1,8 @@

 package interpolation

 import (

-	"fmt"

-

 	"github.com/docker/docker/cli/compose/template"

+	"github.com/pkg/errors"

 )

 // Interpolate replaces variables in a string with the values from a mapping

@@ -15,7 +14,11 @@ func Interpolate(config map[string]interface{}, section string, mapping template

 			out[name] = nil

 			continue

 		}

-		interpolatedItem, err := interpolateSectionItem(name, item.(map[string]interface{}), section, mapping)

+		mapItem, ok := item.(map[string]interface{})

+		if !ok {

+			return nil, errors.Errorf("Invalid type for %s : %T instead of %T", name, item, out)

+		}

+		interpolatedItem, err := interpolateSectionItem(name, mapItem, section, mapping)

 		if err != nil {

 			return nil, err

 		}

@@ -37,7 +40,7 @@ func interpolateSectionItem(

 	for key, value := range item {

 		interpolatedValue, err := recursiveInterpolate(value, mapping)

 		if err != nil {

-			return nil, fmt.Errorf(

+			return nil, errors.Errorf(

 				"Invalid interpolation format for %#v option in %s %#v: %#v. You may need to escape any $ with another $.",

 				key, section, name, err.Template,

 			)