diff --git a/poetry/puzzle/solver.py b/poetry/puzzle/solver.py
index aff19ca6813..e9ea1e80658 100644
--- a/poetry/puzzle/solver.py
+++ b/poetry/puzzle/solver.py
@@ -92,7 +92,7 @@ def solve(self, use_latest=None):  # type: (...) -> List[Operation]
                     elif package.version != pkg.version:
                         # Checking version
                         operations.append(Update(pkg, package))
-                    elif package.source_type != pkg.source_type:
+                    elif pkg.source_type and package.source_type != pkg.source_type:
                         operations.append(Update(pkg, package))
                     else:
                         operations.append(Install(package).skip("Already installed"))
diff --git a/tests/puzzle/test_solver.py b/tests/puzzle/test_solver.py
index 1c75b59862f..6ee6fea911f 100644
--- a/tests/puzzle/test_solver.py
+++ b/tests/puzzle/test_solver.py
@@ -2039,3 +2039,22 @@ def test_solver_cannot_choose_another_version_for_url_dependencies(
     # via the git dependency
     with pytest.raises(SolverProblemError):
         solver.solve()
+
+
+def test_solver_should_not_update_same_version_packages_if_installed_has_no_source_type(
+    solver, repo, package, installed
+):
+    package.add_dependency("foo", "1.0.0")
+
+    foo = get_package("foo", "1.0.0")
+    foo.source_type = "legacy"
+    foo.source_reference = "custom"
+    foo.source_url = "https://foo.bar"
+    repo.add_package(foo)
+    installed.add_package(get_package("foo", "1.0.0"))
+
+    ops = solver.solve()
+
+    check_solver_result(
+        ops, [{"job": "install", "package": foo, "skipped": True}],
+    )