diff --git a/ports/zephyr-cp/Makefile b/ports/zephyr-cp/Makefile
index e21a96e9eb8..aa99eb4d1eb 100644
--- a/ports/zephyr-cp/Makefile
+++ b/ports/zephyr-cp/Makefile
@@ -40,8 +40,11 @@ else
 CP_BOARD_CONF := $(DEBUG_CONF_FILE)
 endif
 endif
+# Quoted: with DEBUG=1 this is a ;-separated list, and an unquoted ; ends the
+# shell command, so west built without debug.conf and the shell then tried to
+# run debug.conf as a program.
 ifneq ($(CP_BOARD_CONF),)
-WEST_CMAKE_ARGS += -Dzephyr-cp_EXTRA_CONF_FILE=$(CP_BOARD_CONF)
+WEST_CMAKE_ARGS += "-Dzephyr-cp_EXTRA_CONF_FILE=$(CP_BOARD_CONF)"
 endif
 
 .PHONY: $(BUILD)/zephyr-cp/zephyr/zephyr.elf flash recover debug debug-jlink debugserver attach run run-sim clean menuconfig all clean-all sim clean-sim test fetch-port-submodules