diff --git a/envd/api/runtime/__init__.py b/envd/api/runtime/__init__.py
index 1b63078cd..c92bda272 100644
--- a/envd/api/runtime/__init__.py
+++ b/envd/api/runtime/__init__.py
@@ -48,8 +48,8 @@ def expose(envd_port: str, host_port: Optional[str], service: Optional[str]):
 
     Args:
         envd_port (str): port in `envd` container
-        host_port (Optional[str]): port in the host, if not provided, `envd` will
-            randomly choose a free port
+        host_port (Optional[str]): port in the host, if not provided or
+            `host_port=0`, `envd` will randomly choose a free port
         service (Optional[str]): service name
     """
 
diff --git a/pkg/lang/frontend/starlark/runtime/runtime.go b/pkg/lang/frontend/starlark/runtime/runtime.go
index 199ada62d..6b5c0a1cd 100644
--- a/pkg/lang/frontend/starlark/runtime/runtime.go
+++ b/pkg/lang/frontend/starlark/runtime/runtime.go
@@ -95,7 +95,7 @@ func ruleFuncExpose(thread *starlark.Thread, _ *starlark.Builtin,
 	args starlark.Tuple, kwargs []starlark.Tuple) (starlark.Value, error) {
 	var (
 		envdPort    starlark.Int
-		hostPort    = starlark.MakeInt(0)
+		hostPort    = starlark.MakeInt(0) // 0 means envd can randomly choose a free port
 		serviceName = starlark.String("")
 	)
 
@@ -108,8 +108,8 @@ func ruleFuncExpose(thread *starlark.Thread, _ *starlark.Builtin,
 		return nil, errors.New("envd_port must be a positive integer less than 65535")
 	}
 	hostPortInt, ok := hostPort.Int64()
-	if !ok || hostPortInt < 1 || hostPortInt > 65535 {
-		return nil, errors.New("envd_port must be a positive integer less than 65535")
+	if !ok || hostPortInt < 0 || hostPortInt > 65535 {
+		return nil, errors.New("host_port must be a positive integer less than 65535")
 	}
 	serviceNameStr := serviceName.GoString()