diff --git a/libavcodec/imgconvert.c b/libavcodec/imgconvert.c
index 7e52ea384d..abc253e96b 100644
--- a/libavcodec/imgconvert.c
+++ b/libavcodec/imgconvert.c
@@ -473,6 +473,8 @@ int ff_set_systematic_pal(uint32_t pal[256], enum PixelFormat pix_fmt){
         case PIX_FMT_GRAY8:
             r=b=g= i;
             break;
+        default:
+            return -1;
         }
         pal[i] =  b + (g<<8) + (r<<16);
     }