autoload/lsp/lsp.vim | 5 +++++

diff --git a/autoload/lsp/lsp.vim b/autoload/lsp/lsp.vim
index edecb3ffba1ef3634aef4983e7951db4e3aa34e6..4b13403f758e6ea6196b669a9cadb67b2706a6c6 100644
--- a/autoload/lsp/lsp.vim
+++ b/autoload/lsp/lsp.vim
@@ -614,6 +614,11 @@     endif
 
     var ProcessDiagHandler: func = null_function
     if server->has_key('processDiagHandler')
+      if server.processDiagHandler->type() != v:t_func
+        util.ErrMsg($'Setting of processDiagHandler {server.processDiagHandler} is not a Funcref nor lambda')
+        return
+      endif
+
       ProcessDiagHandler = server.processDiagHandler
     endif