Error out on negative indices in element()

config/interpolate_funcs.go

@@ -475,7 +475,7 @@ func interpolationFuncElement() ast.Function {
 			list := StringList(args[0].(string))
 
 			index, err := strconv.Atoi(args[1].(string))
-			if err != nil {
+			if err != nil || index < 0 {
 				return "", fmt.Errorf(
 					"invalid number for index, got %s", args[1])
 			}

config/interpolate_funcs_test.go

@@ -778,6 +778,14 @@ func TestInterpolateFuncElement(t *testing.T) {
 				false,
 			},
 
+			// Negative number should fail
+			{
+				fmt.Sprintf(`${element("%s", "-1")}`,
+					NewStringList([]string{"foo"}).String()),
+				nil,
+				true,
+			},
+
 			// Too many args
 			{
 				fmt.Sprintf(`${element("%s", "0", "2")}`,