diff --git a/libs/gfx/source/pixmap.cpp b/libs/gfx/source/pixmap.cpp
index 3027548c..42e30c0b 100644
--- a/libs/gfx/source/pixmap.cpp
+++ b/libs/gfx/source/pixmap.cpp
@@ -119,7 +119,7 @@ namespace psemek::gfx
 	void write_image_pgm(pixmap_monochrome const & p, io::ostream && os)
 	{
 		char header_buffer[256];
-		int header_size = std::snprintf(header_buffer, 256, "P5\n%lu %lu\n255\n", p.width(), p.height());
+		int header_size = std::snprintf(header_buffer, 256, "P5\n%lu %lu\n255\n", (unsigned long)p.width(), (unsigned long)p.height());
 		os.write(header_buffer, header_size);
 		os.write(reinterpret_cast<char const *>(p.data()), p.width() * p.height());
 	}
@@ -127,7 +127,7 @@ namespace psemek::gfx
 	void write_image_ppm(pixmap_rgb const & p, io::ostream && os)
 	{
 		char header_buffer[256];
-		int header_size = std::snprintf(header_buffer, 256, "P6\n%lu %lu\n255\n", p.width(), p.height());
+		int header_size = std::snprintf(header_buffer, 256, "P6\n%lu %lu\n255\n", (unsigned long)p.width(), (unsigned long)p.height());
 		os.write(header_buffer, header_size);
 		os.write(reinterpret_cast<char const *>(p.data()), p.width() * p.height() * 3);
 	}