diff --git a/jsignature/tests/__init__.py b/jsignature/tests/__init__.py
index be2e4bb..29ac698 100644
--- a/jsignature/tests/__init__.py
+++ b/jsignature/tests/__init__.py
@@ -2,3 +2,4 @@ from .widgets import JSignatureWidgetTest
 from .forms import JSignatureFormFieldTest
 from .fields import JSignatureFieldTest
 from .mixins import JSignatureFieldsMixinTest
+from .utils import UtilsTest
diff --git a/jsignature/tests/utils.py b/jsignature/tests/utils.py
new file mode 100644
index 0000000..bf3c827
--- /dev/null
+++ b/jsignature/tests/utils.py
@@ -0,0 +1,32 @@
+import json
+import os
+import imghdr
+from PIL import Image
+from django.test import SimpleTestCase
+
+from ..utils import draw_signature
+
+DUMMY_VALUE = [{"x":[205,210],"y":[59,63]},{"x":[205,207],"y":[67,64]}]
+DUMMY_STR_VALUE = json.dumps(DUMMY_VALUE)
+
+class UtilsTest(SimpleTestCase):
+
+    def test_inputs(self):
+        # Bad str value
+        self.assertRaises(ValueError, draw_signature, 'foo_bar')
+        # Bad type value
+        self.assertRaises(ValueError, draw_signature, object())
+        # Good list value
+        draw_signature(DUMMY_VALUE)
+        # Good str value
+        draw_signature(DUMMY_STR_VALUE)
+
+    def test_outputs(self):
+        # As a file
+        output = draw_signature(DUMMY_VALUE, as_file=True)
+        self.assertTrue(os.path.isfile(output))
+        self.assertIsNotNone(imghdr.what(output))
+        # As an Image
+        output = draw_signature(DUMMY_VALUE)
+        self.assertTrue(issubclass(output.__class__, Image.Image))
+        self.assertTrue(all(output.getbbox()))
diff --git a/jsignature/utils.py b/jsignature/utils.py
index e41a189..320df90 100644
--- a/jsignature/utils.py
+++ b/jsignature/utils.py
@@ -24,8 +24,8 @@ def draw_signature(data, as_file=False):
         raise ValueError
 
     # Compute box
-    width = max(chain(*[d['x'] for d in data])) + 10
-    height = max(chain(*[d['y'] for d in data])) + 10
+    width = max(chain(*[d['x'] for d in drawing])) + 10
+    height = max(chain(*[d['y'] for d in drawing])) + 10
 
     # Draw image
     im = Image.new("RGBA", (width*AA, height*AA))
@@ -45,7 +45,7 @@ def draw_signature(data, as_file=False):
     if as_file:
         ret = im._dump(format='PNG')
     else:
-        ret = img
+        ret = im
 
     return ret