diff --git a/pyproject.toml b/pyproject.toml
index 9330984e..bfdc0f32 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -13,7 +13,7 @@ include = ["resources/**/*"]
 python = "^3.6"
 graphviz = "^0.13.2"
 jinja2 = "^2.10"
-contextvars = "^2.4"
+contextvars = { version = "^2.4", python = "~3.6" }
 
 [tool.poetry.dev-dependencies]
 pytest = "^3.0"