better error handling for type assertions

builtin/provisioners/habitat/resource_provisioner.go

@@ -293,8 +293,13 @@ func validateFn(c *terraform.ResourceConfig) (ws []string, es []error) {
 	// Validate service level configs
 	services, ok := c.Get("service")
 	if ok {
-		for _, svc := range services.([]interface{}) {
-			service := svc.(map[string]interface{})
+		for i, svc := range services.([]interface{}) {
+			service, ok := svc.(map[string]interface{})
+			if !ok {
+				es = append(es, fmt.Errorf("service %d: must be a block", i))
+				continue
+			}
+
 			strategy, ok := service["strategy"].(string)
 			if ok && !updateStrategies[strategy] {
 				es = append(es, errors.New(strategy+" is not a valid update strategy."))