diff --git a/src/mux/muxedit.c b/src/mux/muxedit.c
index e84a89fa..fd79e733 100644
--- a/src/mux/muxedit.c
+++ b/src/mux/muxedit.c
@@ -585,31 +585,55 @@ static WebPMuxError CreateVP8XChunk(WebPMux* const mux) {
   return err;
 }
 
+// Cleans up 'mux' by removing any unnecessary chunks.
+static WebPMuxError MuxCleanup(WebPMux* const mux) {
+  int num_frames;
+  int num_fragments;
+  int num_anim_chunks;
+
+  // If we have an image with single fragment or frame, convert it to a
+  // non-animated non-fragmented image (to avoid writing FRGM/ANMF chunk
+  // unnecessarily).
+  WebPMuxError err = WebPMuxNumChunks(mux, kChunks[IDX_ANMF].id, &num_frames);
+  if (err != WEBP_MUX_OK) return err;
+  err = WebPMuxNumChunks(mux, kChunks[IDX_FRGM].id, &num_fragments);
+  if (err != WEBP_MUX_OK) return err;
+  if (num_frames == 1 || num_fragments == 1) {
+    WebPMuxImage* frame_frag;
+    err = MuxImageGetNth((const WebPMuxImage**)&mux->images_, 1, &frame_frag);
+    assert(err == WEBP_MUX_OK);  // We know that one frame/fragment does exist.
+    if (frame_frag->header_ != NULL) {
+      assert(frame_frag->header_->tag_ == kChunks[IDX_ANMF].tag ||
+             frame_frag->header_->tag_ == kChunks[IDX_FRGM].tag);
+      ChunkDelete(frame_frag->header_);  // Removes ANMF/FRGM chunk.
+      frame_frag->header_ = NULL;
+    }
+    num_frames = 0;
+    num_fragments = 0;
+  }
+  // Remove ANIM chunk if this is a non-animated image.
+  err = WebPMuxNumChunks(mux, kChunks[IDX_ANIM].id, &num_anim_chunks);
+  if (err != WEBP_MUX_OK) return err;
+  if (num_anim_chunks >= 1 && num_frames == 0) {
+    err = MuxDeleteAllNamedData(mux, kChunks[IDX_ANIM].tag);
+    if (err != WEBP_MUX_OK) return err;
+  }
+  return WEBP_MUX_OK;
+}
+
 WebPMuxError WebPMuxAssemble(WebPMux* mux, WebPData* assembled_data) {
   size_t size = 0;
   uint8_t* data = NULL;
   uint8_t* dst = NULL;
-  int num_frames;
-  int num_anim_chunks;
   WebPMuxError err;
 
   if (mux == NULL || assembled_data == NULL) {
     return WEBP_MUX_INVALID_ARGUMENT;
   }
 
-  // Remove ANIM chunk if unnecessary.
-  err = WebPMuxNumChunks(mux, kChunks[IDX_ANIM].id, &num_anim_chunks);
+  // Finalize mux.
+  err = MuxCleanup(mux);
   if (err != WEBP_MUX_OK) return err;
-  if (num_anim_chunks >= 1) {
-    err = WebPMuxNumChunks(mux, kChunks[IDX_ANMF].id, &num_frames);
-    if (err != WEBP_MUX_OK) return err;
-    if (num_frames == 0) {
-      err = MuxDeleteAllNamedData(mux, kChunks[IDX_ANIM].tag);
-      if (err != WEBP_MUX_OK) return err;
-    }
-  }
-
-  // Create VP8X chunk.
   err = CreateVP8XChunk(mux);
   if (err != WEBP_MUX_OK) return err;
 
@@ -641,7 +665,7 @@ WebPMuxError WebPMuxAssemble(WebPMux* mux, WebPData* assembled_data) {
     size = 0;
   }
 
-  // Finalize.
+  // Finalize data.
   assembled_data->bytes = data;
   assembled_data->size = size;