diff --git a/coperator.go b/coperator.go
index d37ea73..f2136f7 100644
--- a/coperator.go
+++ b/coperator.go
@@ -72,14 +72,17 @@ func (d *Document) GetPathArray(path []string) interface{} {
 }
 
 type Collection []Document
-type Filter map[string]interface{}
+type Filter interface{}
 
 type Operator func(doc *Document, filter Filter, key string, value interface{}) (bool, error)
 
 var operatorMap = map[string]Operator{}
 
-func DocumentOperator(doc *Document, filter Filter, key string, value interface{}) (ret bool, err error) {
-
+func DocumentOperator(doc *Document, vfilter Filter, key string, value interface{}) (ret bool, err error) {
+	filter, ok := vfilter.(map[string]interface{})
+	if !ok {
+		return false, errors.New("filter must be a map")
+	}
 	for k, v := range filter {
 
 		if strings.HasPrefix(k, "$") {
@@ -117,7 +120,11 @@ func ValueOperator(doc *Document, filter Filter, key string, value interface{})
 	return true, nil
 }
 
-func FieldOperator(doc *Document, filter Filter, key string, value interface{}) (ret bool, err error) {
+func FieldOperator(doc *Document, vfilter Filter, key string, value interface{}) (ret bool, err error) {
+	filter, ok := vfilter.(map[string]interface{})
+	if !ok {
+		return false, errors.New("filter must be a map")
+	}
 	for k, v := range filter {
 		if !strings.HasPrefix(k, "$") {
 			return false, errors.New("uncorrect grammar")