From 94d0bd53433d88d2e4c3b9e952587b14f44ce166 Mon Sep 17 00:00:00 2001
From: Shaun Cooley <scooley@mapped.com>
Date: Sat, 20 Nov 2021 15:13:40 -0800
Subject: [PATCH] fix: handling of nil passed as an arg to a typed interface
 parameter for #160

---
 handler.go | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/handler.go b/handler.go
index 783823a..aec375f 100644
--- a/handler.go
+++ b/handler.go
@@ -104,7 +104,11 @@ func (h *reflectFunc) fnArgs(msg *Message) ([]reflect.Value, error) {
 			inType := h.ft.In(inStart + i)
 
 			if inType.Kind() == reflect.Interface {
-				if !v.Type().Implements(inType) {
+				// If the input value is an untyped nil, simply create a new
+				// typed nil so that the subsequent .Call() works
+				if !v.IsValid() {
+					v = reflect.Zero(inType)
+				} else if !v.Type().Implements(inType) {
 					hasWrongType = true
 					break
 				}