diff --git a/libavutil/avstring.c b/libavutil/avstring.c
index f03dd25141..4c068f5bc5 100644
--- a/libavutil/avstring.c
+++ b/libavutil/avstring.c
@@ -222,12 +222,13 @@ int av_strcasecmp(const char *a, const char *b)
 
 int av_strncasecmp(const char *a, const char *b, size_t n)
 {
-    const char *end = a + n;
     uint8_t c1, c2;
+    if (n <= 0)
+        return 0;
     do {
         c1 = av_tolower(*a++);
         c2 = av_tolower(*b++);
-    } while (a < end && c1 && c1 == c2);
+    } while (--n && c1 && c1 == c2);
     return c1 - c2;
 }