diff --git a/rpc/api.go b/rpc/api.go
index ff166264b..bde24847f 100644
--- a/rpc/api.go
+++ b/rpc/api.go
@@ -297,10 +297,6 @@ func (api *EthereumApi) GetRequestReply(req *RpcRequest, reply *interface{}) err
 		if err := json.Unmarshal(req.Params, &args); err != nil {
 			return err
 		}
-		if err := args.requirements(); err != nil {
-			return err
-		}
-
 		id := api.xeth().NewFilterString(args.Word)
 		*reply = common.ToHex(big.NewInt(int64(id)).Bytes())
 	case "eth_uninstallFilter":
diff --git a/rpc/args.go b/rpc/args.go
index 459c6546b..96188d02e 100644
--- a/rpc/args.go
+++ b/rpc/args.go
@@ -649,18 +649,13 @@ func (args *FilterStringArgs) UnmarshalJSON(b []byte) (err error) {
 	if !ok {
 		return NewInvalidTypeError("filter", "not a string")
 	}
-	args.Word = argstr
-
-	return nil
-}
-
-func (args *FilterStringArgs) requirements() error {
-	switch args.Word {
+	switch argstr {
 	case "latest", "pending":
 		break
 	default:
 		return NewValidationError("Word", "Must be `latest` or `pending`")
 	}
+	args.Word = argstr
 	return nil
 }
 
diff --git a/rpc/args_test.go b/rpc/args_test.go
index 90b283891..9325b1c9b 100644
--- a/rpc/args_test.go
+++ b/rpc/args_test.go
@@ -1106,10 +1106,6 @@ func TestFilterStringArgs(t *testing.T) {
 		t.Error(err)
 	}
 
-	if err := args.requirements(); err != nil {
-		t.Error(err)
-	}
-
 	if expected.Word != args.Word {
 		t.Errorf("Word shoud be %#v but is %#v", expected.Word, args.Word)
 	}
@@ -1119,9 +1115,39 @@ func TestFilterStringEmptyArgs(t *testing.T) {
 	input := `[]`
 
 	args := new(FilterStringArgs)
-	err := json.Unmarshal([]byte(input), &args)
-	if err == nil {
-		t.Error("Expected error but didn't get one")
+	str := ExpectInsufficientParamsError(json.Unmarshal([]byte(input), &args))
+	if len(str) > 0 {
+		t.Errorf(str)
+	}
+}
+
+func TestFilterStringInvalidArgs(t *testing.T) {
+	input := `{}`
+
+	args := new(FilterStringArgs)
+	str := ExpectDecodeParamError(json.Unmarshal([]byte(input), &args))
+	if len(str) > 0 {
+		t.Errorf(str)
+	}
+}
+
+func TestFilterStringWordInt(t *testing.T) {
+	input := `[7]`
+
+	args := new(FilterStringArgs)
+	str := ExpectInvalidTypeError(json.Unmarshal([]byte(input), &args))
+	if len(str) > 0 {
+		t.Errorf(str)
+	}
+}
+
+func TestFilterStringWordWrong(t *testing.T) {
+	input := `["foo"]`
+
+	args := new(FilterStringArgs)
+	str := ExpectValidationError(json.Unmarshal([]byte(input), &args))
+	if len(str) > 0 {
+		t.Errorf(str)
 	}
 }