diff --git a/libavformat/jacosubdec.c b/libavformat/jacosubdec.c
index 0dd046d213..8103501ff3 100644
--- a/libavformat/jacosubdec.c
+++ b/libavformat/jacosubdec.c
@@ -226,14 +226,17 @@ static int jacosub_read_header(AVFormatContext *s)
             }
             av_bprintf(&header, "#S %s", p);
             break;
-        case 'T': // ...but must be placed after TIMERES
-            jacosub->timeres = strtol(p, NULL, 10);
-            if (!jacosub->timeres)
+        case 'T': { // ...but must be placed after TIMERES
+            int64_t timeres = strtol(p, NULL, 10);
+            if (timeres <= 0 || timeres > UINT32_MAX) {
                 jacosub->timeres = 30;
-            else
+            } else {
+                jacosub->timeres = timeres;
                 av_bprintf(&header, "#T %s", p);
+            }
             break;
         }
+        }
     }
 
     /* general/essential directives in the extradata */