diff --git a/go.mod b/go.mod
index 5937bb8..84a2168 100644
--- a/go.mod
+++ b/go.mod
@@ -1,3 +1,5 @@
 module sourcecode.social/reiver/go-nul
 
 go 1.18
+
+require sourcecode.social/reiver/go-erorr v0.0.0-20230922202459-231149d185a1 // indirect
diff --git a/go.sum b/go.sum
index e69de29..fa79421 100644
--- a/go.sum
+++ b/go.sum
@@ -0,0 +1,2 @@
+sourcecode.social/reiver/go-erorr v0.0.0-20230922202459-231149d185a1 h1:wpnz4JicQBLWrgGphYBls7DysIFCcnWgDz/vce/sY8E=
+sourcecode.social/reiver/go-erorr v0.0.0-20230922202459-231149d185a1/go.mod h1:NFtd7fzEf0r6A6R7JXYZfayRhPaJy0zt/18VWoLzrxA=
diff --git a/nullable_marshaljson.go b/nullable_marshaljson.go
index 62cbacd..bd3fe64 100644
--- a/nullable_marshaljson.go
+++ b/nullable_marshaljson.go
@@ -1,8 +1,9 @@
 package nul
 
 import (
-	"fmt"
 	"encoding/json"
+
+	"sourcecode.social/reiver/go-erorr"
 )
 
 var _ json.Marshaler = Nothing[bool]()
@@ -11,14 +12,14 @@ var _ json.Marshaler = Nothing[string]()
 // MarshalJSON makes it so json.Marshaler is implemented.
 func (receiver Nullable[T]) MarshalJSON() ([]byte, error) {
 	switch interface{}(receiver.value).(type) {
-	case bool, string:
+	case bool, string,json.Marshaler:
 		// these are OK.
 	default:
-		return nil, fmt.Errorf("cannot marshal something of type %T into JSON because parameterized type is ‘%T’ rather than ‘bool’ or ‘string’", receiver, receiver.value)
+		return nil, erorr.Errorf("nul: cannot marshal something of type %T into JSON because parameterized type is ‘%T’ rather than ‘bool’, ‘string’, or ‘json.Marshaler’", receiver, receiver.value)
 	}
 
 	if receiver.isnothing() {
-		return nil, fmt.Errorf("cannot marshal opt.Nothing[%T]() into JSON", receiver.value)
+		return nil, erorr.Errorf("nul: cannot marshal opt.Nothing[%T]() into JSON", receiver.value)
 	}
 
 	if receiver.isnull {
diff --git a/nullable_unmarshaljson.go b/nullable_unmarshaljson.go
index 6155e1e..171f008 100644
--- a/nullable_unmarshaljson.go
+++ b/nullable_unmarshaljson.go
@@ -1,8 +1,9 @@
 package nul
 
 import (
-	"fmt"
 	"encoding/json"
+
+	"sourcecode.social/reiver/go-erorr"
 )
 
 var _ json.Unmarshaler = new(Nullable[bool])
@@ -11,10 +12,10 @@ var _ json.Unmarshaler = new(Nullable[string])
 // UnmarshalJSON makes it so json.Unmarshaler is implemented.
 func (receiver *Nullable[T]) UnmarshalJSON(data []byte) error {
 	switch interface{}(receiver.value).(type) {
-	case bool, string:
+	case bool, string,json.Unmarshaler:
 		// these are OK.
 	default:
-		return fmt.Errorf("cannot unmarshal into something of type %T from JSON because parameterized type is ‘%T’ rather than ‘bool’ or ‘string’", receiver, receiver.value)
+		return erorr.Errorf("nul: cannot unmarshal into something of type %T from JSON because parameterized type is ‘%T’ rather than ‘bool’, ‘string’, or ‘json.Unmarshaler’", receiver, receiver.value)
 	}
 
 	if 4 == len(data) && 'n' == data[0] && 'u' == data[1] && 'l' == data[2] && 'l' == data[3] {