diff --git a/firmware/ice40-riscv/common/spi.h b/firmware/ice40-riscv/common/spi.h
index 3a95358..aab5256 100644
--- a/firmware/ice40-riscv/common/spi.h
+++ b/firmware/ice40-riscv/common/spi.h
@@ -32,5 +32,5 @@
 uint8_t flash_read_sr(void);
 void flash_write_sr(uint8_t sr);
 void flash_read(void *dst, uint32_t addr, unsigned len);
-void flash_page_program(void *src, uint32_t addr, unsigned len);
+void flash_page_program(const void *src, uint32_t addr, unsigned len);
 void flash_sector_erase(uint32_t addr);