diff --git a/go.mod b/go.mod
index d5d00c4..48ed707 100644
--- a/go.mod
+++ b/go.mod
@@ -1,3 +1,5 @@
 module sourcecode.social/reiver/go-opt
 
 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/optional_marshaljson.go b/optional_marshaljson.go
index c2c8111..b802834 100644
--- a/optional_marshaljson.go
+++ b/optional_marshaljson.go
@@ -1,8 +1,9 @@
 package opt
 
 import (
-	"fmt"
 	"encoding/json"
+
+	"sourcecode.social/reiver/go-erorr"
 )
 
 var _ json.Marshaler = Nothing[bool]()
@@ -14,11 +15,11 @@ func (receiver Optional[T]) MarshalJSON() ([]byte, error) {
 	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("opt: 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("opt: cannot marshal opt.Nothing[%T]() into JSON", receiver.value)
 	}
 
 	return json.Marshal(receiver.value)
diff --git a/optional_unmarshaljson.go b/optional_unmarshaljson.go
index 51e6a1c..7f7c8cd 100644
--- a/optional_unmarshaljson.go
+++ b/optional_unmarshaljson.go
@@ -1,8 +1,9 @@
 package opt
 
 import (
-	"fmt"
 	"encoding/json"
+
+	"sourcecode.social/reiver/go-erorr"
 )
 
 var _ json.Unmarshaler = new(Optional[bool])
@@ -14,7 +15,7 @@ func (receiver *Optional[T]) UnmarshalJSON(data []byte) error {
 	case *bool, *string, json.Unmarshaler:
 		// these are OK.
 	default:
-		return fmt.Errorf("cannot unmarshal into something of type %T from JSON because pointer to parameterized type is ‘%T’ rather than ‘*bool’, ‘*string’, or ‘json.Unmarshaler’", receiver, &receiver.value)
+		return erorr.Errorf("opt: cannot unmarshal into something of type %T from JSON because pointer to parameterized type is ‘%T’ rather than ‘*bool’, ‘*string’, or ‘json.Unmarshaler’", receiver, &receiver.value)
 	}
 
 	{