diff --git a/gitstatus/mbuild b/gitstatus/mbuild
index 40316fdf..d3de3317 100755
--- a/gitstatus/mbuild
+++ b/gitstatus/mbuild
@@ -94,7 +94,9 @@
 setopt no_unset extended_glob pipe_fail prompt_percent typeset_silent \
   no_prompt_subst no_prompt_bang pushd_silent warn_create_global
 
-if [[ $ZSH_VERSION != (5.<1->*|<6->.*) || $ZSH_VERSION == 5.4(|.*) ]]; then
+autoload -Uz is-at-least
+
+if ! is-at-least 5.1 || [[ $ZSH_VERSION == 5.4.* ]]; then
   print -ru2 -- "[error] unsupported zsh version: $ZSH_VERSION"
   return 1
 fi
diff --git a/gitstatus/src/arena.h b/gitstatus/src/arena.h
index 569833ca..96873852 100644
--- a/gitstatus/src/arena.h
+++ b/gitstatus/src/arena.h
@@ -27,6 +27,7 @@
 #include <type_traits>
 #include <vector>
 
+#include "check.h"
 #include "string_view.h"
 
 namespace gitstatus {
@@ -94,6 +95,7 @@ class Arena {
   template <class T>
   inline T* Allocate(size_t n) {
     static_assert(!std::is_reference<T>(), "");
+    CHECK(n <= SIZE_MAX / sizeof(T)) << n;
     return static_cast<T*>(Allocate(n * sizeof(T), alignof(T)));
   }