diff --git a/poetry/installation/pip_installer.py b/poetry/installation/pip_installer.py
index a5227cc6928..d4702565c2e 100644
--- a/poetry/installation/pip_installer.py
+++ b/poetry/installation/pip_installer.py
@@ -142,7 +142,7 @@ def requirement(self, package, formatted=False):
 
         if package.source_type in ["file", "directory"]:
             if package.root_dir:
-                req = os.path.join(package.root_dir, package.source_url)
+                req = (package.root_dir / package.source_url).as_posix()
             else:
                 req = os.path.realpath(package.source_url)
 
@@ -185,7 +185,7 @@ def install_directory(self, package):
         from poetry.utils.toml_file import TomlFile
 
         if package.root_dir:
-            req = os.path.join(package.root_dir, package.source_url)
+            req = (package.root_dir / package.source_url).as_posix()
         else:
             req = os.path.realpath(package.source_url)