@@ -240,12 +240,18 @@ func interpolationFuncConcat() ast.Function {
 				// Otherwise variables
 				if argument, ok := arg.([]ast.Variable); ok {
 					for _, element := range argument {
-						finalListElements = append(finalListElements, element.Value.(string))
+						t := element.Type
+						switch t {
+						case ast.TypeString:
+							finalListElements = append(finalListElements, element.Value.(string))
+						default:
+							return nil, fmt.Errorf("concat() does not support lists of %s", t.Printable())
+						}
 					}
 					continue
 				}
 
-				return nil, fmt.Errorf("arguments to concat() must be a string or list")
+				return nil, fmt.Errorf("arguments to concat() must be a string or list of strings")
 			}
 
 			return stringSliceToVariableValue(finalListElements), nil
 
@@ -5,6 +5,7 @@ import (
 	"io/ioutil"
 	"os"
 	"reflect"
+	"strings"
 	"testing"
 
 	"github.com/hashicorp/hil"
@@ -226,6 +227,40 @@ func TestInterpolateFuncConcat(t *testing.T) {
 	})
 }
 
+// TODO: This test is split out and calls a private function
+// because there's no good way to get a list of maps into the unit
+// tests due to GH-7142 - once lists of maps can be expressed properly as
+// literals this unit test can be wrapped back into the suite above.
+//
+// Reproduces crash reported in GH-7030.
+func TestInterpolationFuncConcatListOfMaps(t *testing.T) {
+	listOfMapsOne := ast.Variable{
+		Type: ast.TypeList,
+		Value: []ast.Variable{
+			{
+				Type:  ast.TypeMap,
+				Value: map[string]interface{}{"one": "foo"},
+			},
+		},
+	}
+	listOfMapsTwo := ast.Variable{
+		Type: ast.TypeList,
+		Value: []ast.Variable{
+			{
+				Type:  ast.TypeMap,
+				Value: map[string]interface{}{"two": "bar"},
+			},
+		},
+	}
+	args := []interface{}{listOfMapsOne.Value, listOfMapsTwo.Value}
+
+	_, err := interpolationFuncConcat().Callback(args)
+
+	if err == nil || !strings.Contains(err.Error(), "concat() does not support lists of type map") {
+		t.Fatalf("Expected err, got: %v", err)
+	}
+}
+
 func TestInterpolateFuncFile(t *testing.T) {
 	tf, err := ioutil.TempFile("", "tf")
 	if err != nil {
Original file line number	Diff line number	Diff line change
`@@ -240,12 +240,18 @@ func interpolationFuncConcat() ast.Function {`
`240`	`240`	`// Otherwise variables`
`241`	`241`	`if argument, ok := arg.([]ast.Variable); ok {`
`242`	`242`	`for _, element := range argument {`
`243`		`- finalListElements = append(finalListElements, element.Value.(string))`
	`243`	`+ t := element.Type`
	`244`	`+ switch t {`
	`245`	`+ case ast.TypeString:`
	`246`	`+ finalListElements = append(finalListElements, element.Value.(string))`
	`247`	`+ default:`
	`248`	`+ return nil, fmt.Errorf("concat() does not support lists of %s", t.Printable())`
	`249`	`+ }`
`244`	`250`	`}`
`245`	`251`	`continue`
`246`	`252`	`}`
`247`	`253`
`248`		`- return nil, fmt.Errorf("arguments to concat() must be a string or list")`
	`254`	`+ return nil, fmt.Errorf("arguments to concat() must be a string or list of strings")`
`249`	`255`	`}`
`250`	`256`
`251`	`257`	`return stringSliceToVariableValue(finalListElements), nil`