diff --git a/bitarray/_bitarray.c b/bitarray/_bitarray.c
index 1d3ab86d..4a2e339a 100644
--- a/bitarray/_bitarray.c
+++ b/bitarray/_bitarray.c
@@ -574,11 +574,18 @@ extend_bitarray(bitarrayobject *self, bitarrayobject *other)
     if (other->nbits == 0)
         return 0;
 
+    /*
+        Note: other may be self. Thus we take the size before we change the
+        size, ensuring we only copy the right parts of the array.
+    */
+
+    idx_t n_other_bits = other->nbits;
+
     n_sum = self->nbits + other->nbits;
     if (resize(self, n_sum) < 0)
         return -1;
 
-    copy_n(self, n_sum - other->nbits, other, 0, other->nbits);
+    copy_n(self, n_sum - other->nbits, other, 0, n_other_bits);
     return 0;
 }
 
diff --git a/bitarray/test_bitarray.py b/bitarray/test_bitarray.py
index 44de2f02..63b98765 100644
--- a/bitarray/test_bitarray.py
+++ b/bitarray/test_bitarray.py
@@ -1211,6 +1211,11 @@ def test_string01(self):
                 self.assertEqual(c.tolist(), a + b)
                 self.check_obj(c)
 
+    def test_extend_self(self):
+        a = bitarray('1')
+        a.extend(a)
+        self.assertEqual(a, bitarray('11'))
+
 
 tests.append(ExtendTests)