From 6000855c82fc1bdc5c256480b4161511f91ed703 Mon Sep 17 00:00:00 2001
From: Matthew Holt <mholt@users.noreply.github.com>
Date: Wed, 26 Jun 2019 10:45:34 -0600
Subject: [PATCH] Fix panics by disallowing explicitly-defined null modules in
 config

---
 context.go | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/context.go b/context.go
index 2af38ff60..9661aca4d 100644
--- a/context.go
+++ b/context.go
@@ -100,6 +100,15 @@ func (ctx Context) LoadModule(name string, rawMsg json.RawMessage) (interface{},
 		}
 	}
 
+	if val == nil {
+		// returned module values are almost always type-asserted
+		// before being used, so a nil value would panic; and there
+		// is no good reason to explicitly declare null modules in
+		// a config; it might be because the user is trying to
+		// achieve a result they aren't expecting, which is a smell
+		return nil, fmt.Errorf("module value cannot be null")
+	}
+
 	if prov, ok := val.(Provisioner); ok {
 		err := prov.Provision(ctx)
 		if err != nil {