diff --git a/img_png.h b/img_png.h
index 18b39d8..20fbb03 100644
--- a/img_png.h
+++ b/img_png.h
@@ -3,7 +3,7 @@
 
 
 struct img_png_chunk {
-  char* chunk_data;
+  unsigned char* chunk_data;
   uint32_t length;
   uint32_t crc;
   char chunk_type[4];