UnmarshalJSON in null.Bool, null.Float, and null.String now return an error message when asked to unmarshal from an invalid type. This is in line with the behavior of the built-in sql library's null values.

bool.go

@@ -4,7 +4,8 @@ import (
 	"database/sql"
 	"encoding/json"
 	"errors"
-	// "fmt"
+	"fmt"
+	"reflect"
 )
 
 // Bool is an even nuller nullable bool.
@@ -53,8 +54,8 @@ func (b *Bool) UnmarshalJSON(data []byte) error {
 	case nil:
 		b.Valid = false
 		return nil
-		// default:
+	default:
-		// 	err = fmt.Errorf("json: cannot unmarshal %v into Go value of type null.Bool", v.(type).Name())
+		err = fmt.Errorf("json: cannot unmarshal %v into Go value of type null.Bool", reflect.TypeOf(v).Name())
 	}
 	b.Valid = err == nil
 	return err

float.go

@@ -3,6 +3,8 @@ package null
 import (
 	"database/sql"
 	"encoding/json"
+	"fmt"
+	"reflect"
 	"strconv"
 )
 
@@ -52,6 +54,8 @@ func (f *Float) UnmarshalJSON(data []byte) error {
 	case nil:
 		f.Valid = false
 		return nil
+	default:
+		err = fmt.Errorf("json: cannot unmarshal %v into Go value of type null.Float", reflect.TypeOf(v).Name())
 	}
 	f.Valid = err == nil
 	return err

string.go

@@ -6,6 +6,8 @@ package null
 import (
 	"database/sql"
 	"encoding/json"
+	"fmt"
+	"reflect"
 )
 
 // String is an even nuller nullable string.
@@ -51,6 +53,8 @@ func (s *String) UnmarshalJSON(data []byte) error {
 	case nil:
 		s.Valid = false
 		return nil
+	default:
+		err = fmt.Errorf("json: cannot unmarshal %v into Go value of type null.String", reflect.TypeOf(v).Name())
 	}
 	s.Valid = (err == nil) && (s.String != "")
 	return err