Panic: interface{} is nil, not string

[turt2live]

2018/07/15 23:51:19 INFO: Webhook received
2018/07/15 23:51:19 INFO: Parsing Payload...
2018/07/15 23:51:19 INFO: Checking secret
panic: interface conversion: interface {} is nil, not string

goroutine 7 [running]:
main.startMatrix.func1(0xc42001a0e0)
        /home/gitlab/maulabbot/matrix.go:58 +0x792
created by main.startMatrix
        /home/gitlab/maulabbot/matrix.go:48 +0x420

Version: 7ea206f with a minor patch:

diff --git a/matrix.go b/matrix.go
index f337a8f..ac04fb8 100644
--- a/matrix.go
+++ b/matrix.go
@@ -54,7 +54,7 @@ func startMatrix() func() {
                        case evt := <-mxbot.Timeline:
                                switch evt.Type {
                                case mautrix.EvtRoomMessage:
-                                       evt.MarkRead()
+                                       //evt.MarkRead()
                                        msg := evt.Content["body"].(string)
                                        if !strings.HasPrefix(msg, "!gitlab") {
                                                continue Loop