diff --git a/src/nox_poetry/sessions.py b/src/nox_poetry/sessions.py
index 538125fa..1c73174b 100644
--- a/src/nox_poetry/sessions.py
+++ b/src/nox_poetry/sessions.py
@@ -59,7 +59,7 @@ def to_constraint(requirement_string: str, line: int) -> Optional[str]:
     """Convert requirement to constraint."""
     if any(
         requirement_string.startswith(prefix)
-        for prefix in ("-e ", "file://", "git+https://", "http://", "https://")
+        for prefix in ("-", "file://", "git+https://", "http://", "https://")
     ):
         return None
 
diff --git a/tests/unit/test_sessions.py b/tests/unit/test_sessions.py
index 1f3ecf22..a1994585 100644
--- a/tests/unit/test_sessions.py
+++ b/tests/unit/test_sessions.py
@@ -152,6 +152,11 @@ def test_session_build_package(proxy: nox_poetry.Session) -> None:
             "regex==2020.10.28; python_version == '3.5'",
             'regex==2020.10.28; python_version == "3.5"',
         ),
+        ("-e ../lib/foo", ""),
+        (
+            "--extra-index-url https://example.com/pypi/simple",
+            "",
+        ),
     ],
 )
 def test_to_constraints(requirements: str, expected: str) -> None: