diff --git a/examples/pngdec.c b/examples/pngdec.c
index 59e8b77d..332f6bbd 100644
--- a/examples/pngdec.c
+++ b/examples/pngdec.c
@@ -26,8 +26,8 @@
 #include "webp/encode.h"
 #include "./metadata.h"
 
-static void PNGAPI error_function(png_structp png, png_const_charp dummy) {
-  (void)dummy;  // remove variable-unused warning
+static void PNGAPI error_function(png_structp png, png_const_charp error) {
+  if (error != NULL) fprintf(stderr, "libpng error: %s\n", error);
   longjmp(png_jmpbuf(png), 1);
 }