diff --git a/src/core/ngx_string.c b/src/core/ngx_string.c
index fbde1c2ce..8f2a17159 100644
--- a/src/core/ngx_string.c
+++ b/src/core/ngx_string.c
@@ -703,6 +703,10 @@ ngx_strlcasestrn(u_char *s1, u_char *last, u_char *s2, size_t n)
 {
     ngx_uint_t  c1, c2;
 
+    if (s1 <= last) {
+        return NULL;
+    }
+
     c2 = (ngx_uint_t) *s2++;
     c2  = (c2 >= 'A' && c2 <= 'Z') ? (c2 | 0x20) : c2;
     last -= n;