Fix an error when locking packages with no description

CHANGELOG.md

@@ -7,6 +7,7 @@
 - Fixed the way packages information are retrieved for legacy repositories.
 - Fixed an error when adding packages with invalid versions.
 - Fixed an error when resolving directory dependencies with no sub dependencies.
+- Fixed an error when locking packages with no description.
 
 
 ## [0.12.10] - 2018-11-22

poetry/packages/locker.py

@@ -218,7 +218,7 @@ class Locker:
         data = {
             "name": package.pretty_name,
             "version": package.pretty_version,
-            "description": package.description,
+            "description": package.description or "",
             "category": package.category,
             "optional": package.optional,
             "python-versions": package.python_versions,

tests/packages/test_locker.py

@@ -61,3 +61,31 @@ B = []
 """
 
     assert expected == content
+
+
+def test_lock_packages_with_null_description(locker, root):
+    package_a = get_package("A", "1.0.0")
+    package_a.description = None
+
+    locker.set_lock_data(root, [package_a])
+
+    with locker.lock.open(encoding="utf-8") as f:
+        content = f.read()
+
+    expected = """[[package]]
+category = "main"
+description = ""
+name = "A"
+optional = false
+python-versions = "*"
+version = "1.0.0"
+
+[metadata]
+content-hash = "115cf985d932e9bf5f540555bbdd75decbb62cac81e399375fc19f6277f8c1d8"
+python-versions = "*"
+
+[metadata.hashes]
+A = []
+"""
+
+    assert expected == content