diff --git a/jsonfield/fields.py b/jsonfield/fields.py
index 009251c..55f5c45 100644
--- a/jsonfield/fields.py
+++ b/jsonfield/fields.py
@@ -7,6 +7,7 @@
 
 from . import forms
 from .encoder import JSONEncoder
+from .json import checked_loads
 
 DEFAULT_DUMP_KWARGS = {
     'cls': JSONEncoder,
@@ -36,14 +37,8 @@ def deconstruct(self):
         return name, path, args, kwargs
 
     def to_python(self, value):
-        if self.null and value is None:
-            return None
-
-        if not isinstance(value, (str, bytes, bytearray)):
-            return value
-
         try:
-            return json.loads(value, **self.load_kwargs)
+            return checked_loads(value, **self.load_kwargs)
         except ValueError:
             raise ValidationError(_("Enter valid JSON."))
 
@@ -58,12 +53,6 @@ def get_prep_value(self, value):
             return None
         return json.dumps(value, **self.dump_kwargs)
 
-    def value_from_object(self, obj):
-        value = super(JSONFieldMixin, self).value_from_object(obj)
-        if self.null and value is None:
-            return None
-        return json.dumps(value, **self.dump_kwargs)
-
     def formfield(self, **kwargs):
         if 'form_class' not in kwargs:
             kwargs['form_class'] = self.form_class
diff --git a/jsonfield/forms.py b/jsonfield/forms.py
index fe9c403..ab30b63 100644
--- a/jsonfield/forms.py
+++ b/jsonfield/forms.py
@@ -3,6 +3,8 @@
 from django.forms import ValidationError, fields
 from django.utils.translation import gettext_lazy as _
 
+from .json import checked_loads
+
 
 class JSONField(fields.CharField):
     def __init__(self, *args, dump_kwargs=None, load_kwargs=None, **kwargs):
@@ -12,19 +14,16 @@ def __init__(self, *args, dump_kwargs=None, load_kwargs=None, **kwargs):
         super().__init__(*args, **kwargs)
 
     def to_python(self, value):
-        if isinstance(value, str) and value:
-            try:
-                return json.loads(value, **self.load_kwargs)
-            except ValueError:
-                raise ValidationError(_("Enter valid JSON."))
-        return value
-
-    def clean(self, value):
-        if not value and not self.required:
+        if self.disabled:
+            return value
+
+        if value in self.empty_values:
             return None
 
-        # Trap cleaning errors & bubble them up as JSON errors
         try:
-            return super().clean(value)
-        except TypeError:
+            return checked_loads(value, **self.load_kwargs)
+        except json.JSONDecodeError:
             raise ValidationError(_("Enter valid JSON."))
+
+    def prepare_value(self, value):
+        return json.dumps(value, **self.dump_kwargs)
diff --git a/jsonfield/json.py b/jsonfield/json.py
new file mode 100644
index 0000000..86f1736
--- /dev/null
+++ b/jsonfield/json.py
@@ -0,0 +1,22 @@
+import json
+
+
+class JSONString(str):
+    pass
+
+
+def checked_loads(value, **kwargs):
+    """
+    Ensure that values aren't loaded twice, resulting in an encoding error.
+
+    Loaded strings are wrapped in JSONString, as it is otherwise not possible
+    to differentiate between a loaded and unloaded string.
+    """
+    if isinstance(value, (list, dict, int, float, JSONString, type(None))):
+        return value
+
+    value = json.loads(value, **kwargs)
+    if isinstance(value, str):
+        value = JSONString(value)
+
+    return value