diff --git a/libavcodec/pnm.c b/libavcodec/pnm.c
index 680034a..be445d1 100644
--- a/libavcodec/pnm.c
+++ b/libavcodec/pnm.c
@@ -124,7 +124,10 @@ int ff_pnm_decode_header(AVCodecContext *avctx, PNMContext * const s)
                 return -1;
             }
         } else if (depth == 4) {
-            avctx->pix_fmt = PIX_FMT_RGB32;
+            if (maxval < 256)
+                avctx->pix_fmt = PIX_FMT_RGB32;
+            else
+                return -1;
         } else {
             return -1;
         }