diff --git a/boot/zephyr/Kconfig b/boot/zephyr/Kconfig
index 34cb541..2d2e05e 100644
--- a/boot/zephyr/Kconfig
+++ b/boot/zephyr/Kconfig
@@ -36,6 +36,10 @@
 	bool
 	# Hidden option
 	default n
+	# When building for ECDSA, we use our own copy of mbedTLS, so the
+	# Zephyr one must not be enabled or the MBEDTLS_CONFIG_FILE macros
+	# will collide.
+	depends on ! MBEDTLS
 	help
 	  Use TinyCrypt for crypto primitives.