diff --git a/ports/atmel-samd/mpconfigport.h b/ports/atmel-samd/mpconfigport.h
index 20f5e2bed0..686bdcd294 100644
--- a/ports/atmel-samd/mpconfigport.h
+++ b/ports/atmel-samd/mpconfigport.h
@@ -92,11 +92,13 @@
 
 #ifdef SAMD21
 
+#ifndef CIRCUITPY_INTERNAL_FLASH_FILESYSTEM_SIZE
 #if INTERNAL_FLASH_FILESYSTEM
 #define CIRCUITPY_INTERNAL_FLASH_FILESYSTEM_SIZE (64 * 1024)
 #else
 #define CIRCUITPY_INTERNAL_FLASH_FILESYSTEM_SIZE (0)
 #endif
+#endif
 
 #ifndef CIRCUITPY_INTERNAL_NVM_SIZE
 #define CIRCUITPY_INTERNAL_NVM_SIZE (256)