diff --git a/sk1/compressor.c b/sk1/compressor.c
index f4ac350..f1f9f59 100644
--- a/sk1/compressor.c
+++ b/sk1/compressor.c
@@ -49,7 +49,7 @@ int compress_1(const char* input_file_name, const char* output_file_name) {
         while (i + count < size && data[i + count] == byte && count < 255) {
             count++;
         }
-        fprintf(output, "%c%d", byte, (int)count);
+        fprintf(output, "%c%zu", byte, (int)count);
 
         i += count;
     }