diff --git a/src/enc/vp8enci.h b/src/enc/vp8enci.h
index f5e68877..2900db51 100644
--- a/src/enc/vp8enci.h
+++ b/src/enc/vp8enci.h
@@ -473,7 +473,7 @@ int VP8StatLoop(VP8Encoder* const enc);
 
   // in webpenc.c
 // Assign an error code to a picture. Return false for convenience.
-int WebPEncodingSetError(WebPPicture* const pic, WebPEncodingError error);
+int WebPEncodingSetError(const WebPPicture* const pic, WebPEncodingError error);
 int WebPReportProgress(VP8Encoder* const enc, int percent);
 
   // in analysis.c
diff --git a/src/enc/webpenc.c b/src/enc/webpenc.c
index 92c83c08..bbf604d4 100644
--- a/src/enc/webpenc.c
+++ b/src/enc/webpenc.c
@@ -306,10 +306,11 @@ static void StoreStats(VP8Encoder* const enc) {
   WebPReportProgress(enc, 100);  // done!
 }
 
-int WebPEncodingSetError(WebPPicture* const pic, WebPEncodingError error) {
+int WebPEncodingSetError(const WebPPicture* const pic,
+                         WebPEncodingError error) {
   assert((int)error < VP8_ENC_ERROR_LAST);
   assert((int)error >= VP8_ENC_OK);
-  pic->error_code = error;
+  ((WebPPicture*)pic)->error_code = error;
   return 0;
 }