diff --git a/django/db/models/functions.py b/django/db/models/functions.py
index f16cf10b15..610ecb6985 100644
--- a/django/db/models/functions.py
+++ b/django/db/models/functions.py
@@ -110,13 +110,13 @@ class Substr(Func):
         pos: an integer > 0, or an expression returning an integer
         length: an optional number of characters to return
         """
-        if not hasattr('pos', 'resolve_expression'):
+        if not hasattr(pos, 'resolve_expression'):
             if pos < 1:
                 raise ValueError("'pos' must be greater than 0")
             pos = Value(pos)
         expressions = [expression, pos]
         if length is not None:
-            if not hasattr('length', 'resolve_expression'):
+            if not hasattr(length, 'resolve_expression'):
                 length = Value(length)
             expressions.append(length)
         super(Substr, self).__init__(*expressions, **extra)
diff --git a/tests/db_functions/tests.py b/tests/db_functions/tests.py
index 00a5e258de..97d32d86e7 100644
--- a/tests/db_functions/tests.py
+++ b/tests/db_functions/tests.py
@@ -278,6 +278,18 @@ class FunctionTests(TestCase):
         with six.assertRaisesRegex(self, ValueError, "'pos' must be greater than 0"):
             Author.objects.annotate(raises=Substr('name', 0))
 
+    def test_substr_with_expressions(self):
+        Author.objects.create(name='John Smith', alias='smithj')
+        Author.objects.create(name='Rhonda')
+        authors = Author.objects.annotate(name_part=Substr('name', V(5), V(3)))
+        self.assertQuerysetEqual(
+            authors.order_by('name'), [
+                ' Sm',
+                'da',
+            ],
+            lambda a: a.name_part
+        )
+
     def test_nested_function_ordering(self):
         Author.objects.create(name='John Smith')
         Author.objects.create(name='Rhonda Simpson', alias='ronny')