diff --git a/src/event/ngx_event_openssl.c b/src/event/ngx_event_openssl.c
index 68d02bf..cddcefd 100644
--- a/src/event/ngx_event_openssl.c
+++ b/src/event/ngx_event_openssl.c
@@ -2137,7 +2137,9 @@
             break;
         }
 
-        if (p >= last) {
+        /* ERR_error_string_n() requires at least one byte */
+
+        if (p >= last - 1) {
             goto next;
         }