diff --git a/SDWebImageAVIFCoder/Classes/SDImageAVIFCoder.m b/SDWebImageAVIFCoder/Classes/SDImageAVIFCoder.m
index 6036938..a0111e4 100644
--- a/SDWebImageAVIFCoder/Classes/SDImageAVIFCoder.m
+++ b/SDWebImageAVIFCoder/Classes/SDImageAVIFCoder.m
@@ -348,6 +348,7 @@ - (UIImage *)animatedImageFrameAtIndex:(NSUInteger)index {
     SD_LOCK(_lock);
     avifResult decodeResult = avifDecoderNthImage(_decoder, (uint32_t)index);
     if (decodeResult != AVIF_RESULT_OK) {
+        SD_UNLOCK(_lock);
         return nil;
     }
     CGImageRef imageRef = SDCreateCGImageFromAVIF(_decoder->image);