diff --git a/errors.go b/errors.go
new file mode 100644
index 0000000..fcf57ad
--- /dev/null
+++ b/errors.go
@@ -0,0 +1,12 @@
+package filter
+
+import "errors"
+
+var (
+	errEmpty = newErr("empty query")
+	nilErr   = errors.New("nil")
+)
+
+func newErr(s string) error {
+	return errors.New("filter: " + s)
+}
diff --git a/filter.go b/filter.go
index fe7d453..0025946 100644
--- a/filter.go
+++ b/filter.go
@@ -4,7 +4,6 @@
 package filter
 
 import (
-	"errors"
 	"reflect"
 	"sort"
 	"strings"
@@ -97,13 +96,13 @@ type Query struct {
 func NewQuery(s string) (q *Query, err error) {
 	q = &Query{q: s}
 	if s == "" {
-		err = errors.New("query is empty")
+		err = errEmpty
 		return
 	}
 	qm := make(map[string]struct{})
 	fields := clearFields(s)
 	if len(fields) == 0 {
-		err = errors.New("query is empty")
+		err = errEmpty
 		return
 	}
 	for _, v := range fields {
@@ -132,7 +131,7 @@ func NewQuery(s string) (q *Query, err error) {
 						}
 						continue
 					default:
-						err = errors.New("invalid sort: " + v)
+						err = newErr("invalid sort: " + v)
 						return
 					}
 
@@ -141,12 +140,12 @@ func NewQuery(s string) (q *Query, err error) {
 		}
 		m := strings.SplitN(v, ":", 2)
 		if len(m) != 2 {
-			err = errors.New("invalid match: " + v)
+			err = newErr("invalid match: " + v)
 			return
 		}
 		if q.l == and {
 			if _, ok := qm[m[0]]; ok {
-				err = errors.New("field for AND matching used more than once: " + m[0])
+				err = newErr("field for AND matching used more than once: " + m[0])
 				return
 			}
 			qm[m[0]] = struct{}{}
@@ -164,7 +163,7 @@ func NewQuery(s string) (q *Query, err error) {
 	if len(q.m) == 0 && q.s.enabled {
 		q.s.only = true
 	} else if len(q.m) == 0 {
-		err = errors.New("query is empty")
+		err = errEmpty
 	}
 	if len(q.m) == 1 && q.l != not {
 		q.l = or
@@ -281,7 +280,7 @@ func (q Query) sort(f Filter) {
 func (q Query) Filter(f Filter) error {
 	v := reflect.ValueOf(f)
 	if v.Kind() != reflect.Ptr {
-		return errors.New("filter: non-pointer " + v.Type().String())
+		return newErr("non-pointer " + v.Type().String())
 	}
 
 	if q.s.enabled && q.s.only {
diff --git a/helper.go b/helper.go
index 7a350f1..4bd92a7 100644
--- a/helper.go
+++ b/helper.go
@@ -83,7 +83,7 @@ func getValue(v reflect.Value) (s string, b bool, err error) {
 	case reflect.Bool:
 		b = v.Bool()
 	default:
-		err = errors.New("nil")
+		err = nilErr
 	}
 	return
 }