diff --git a/ports/cxd56/mpconfigport.mk b/ports/cxd56/mpconfigport.mk
index 33a993ad95..e767c6326f 100644
--- a/ports/cxd56/mpconfigport.mk
+++ b/ports/cxd56/mpconfigport.mk
@@ -10,8 +10,14 @@ USB_MSC_EP_NUM_IN = 4
 # Number of USB endpoint pairs.
 USB_NUM_EP = 6
 
+# Define an equivalent for MICROPY_LONGINT_IMPL, to pass to $(MPY-TOOL) in py/mkrules.mk
+# $(MPY-TOOL) needs to know what kind of longint to use (if any) to freeze long integers.
+# This should correspond to the MICROPY_LONGINT_IMPL definition in mpconfigport.h.
 MPY_TOOL_LONGINT_IMPL = -mlongint-impl=mpz
 
+# Longints can be implemented as mpz, as longlong, or not
+LONGINT_IMPL = MPZ
+
 CIRCUITPY_AUDIOBUSIO = 0
 CIRCUITPY_AUDIOIO = 0
 CIRCUITPY_CAMERA = 1