diff --git a/libavutil/log.c b/libavutil/log.c
index 56ef53b378..5923228039 100644
--- a/libavutil/log.c
+++ b/libavutil/log.c
@@ -200,7 +200,7 @@ static void format_line(void *ptr, int level, const char *fmt, va_list vl,
     av_vbprintf(part+2, fmt, vl);
 
     if(*part[0].str || *part[1].str || *part[2].str) {
-        char lastc = part[2].len ? part[2].str[part[2].len - 1] : 0;
+        char lastc = part[2].len && part[2].len <= part[2].size ? part[2].str[part[2].len - 1] : 0;
         *print_prefix = lastc == '\n' || lastc == '\r';
     }
 }