diff --git a/apisix/core/env.lua b/apisix/core/env.lua
index 4d758ae6a9dd..09ef41d7da85 100644
--- a/apisix/core/env.lua
+++ b/apisix/core/env.lua
@@ -59,7 +59,9 @@ end
 
 
 local function is_env_ref(ref)
-    return type(ref) == "string" and sub(upper(ref), 1, 7) == ENV_PREFIX
+    -- Avoid the error caused by has_prefix to cause a crash,
+    -- so judge the type of ref.
+    return type(ref) == "string" and string.has_prefix(upper(ref), ENV_PREFIX)
 end
 
 
diff --git a/apisix/core/utils.lua b/apisix/core/utils.lua
index 2f6f6aab03e5..d74c48bc75e1 100644
--- a/apisix/core/utils.lua
+++ b/apisix/core/utils.lua
@@ -35,6 +35,7 @@ local base           = require("resty.core.base")
 local open           = io.open
 local sub_str        = string.sub
 local str_byte       = string.byte
+local str_format     = string.format
 local tonumber       = tonumber
 local tostring       = tostring
 local pairs          = pairs
@@ -352,7 +353,7 @@ do
     end
 
     local function retrieve(refs)
-        log.info(string.format("retrieve secrets refs: %p", refs))
+        log.info(str_format("retrieve secrets refs: %p", refs))
 
         local new_refs = table.deepcopy(refs)
         return retrieve_ref(new_refs)