diff --git a/src/mux/anim_encode.c b/src/mux/anim_encode.c
index 4b1a1120..66ccbb8c 100644
--- a/src/mux/anim_encode.c
+++ b/src/mux/anim_encode.c
@@ -1262,10 +1262,13 @@ static int CacheFrame(WebPAnimEncoder* const enc,
       // Note: We need '>=' below because when kmin and kmax are both zero,
       // count_since_key_frame will always be > kmax.
       if (enc->count_since_key_frame >= enc->options.kmax) {
+        // Force the next frame to be a key frame.
+        enc->best_delta = DELTA_INFINITY;
+        // Flush all encoded frames.
         enc->flush_count = enc->count - 1;
         enc->count_since_key_frame = 0;
+        enc->prev_candidate_undecided = 0;
         enc->keyframe = KEYFRAME_NONE;
-        enc->best_delta = DELTA_INFINITY;
       }
       if (!enc->prev_candidate_undecided) {
         enc->prev_rect =