diff --git a/toolchain/toolchain-external/toolchain-external-custom/toolchain-external-custom.mk b/toolchain/toolchain-external/toolchain-external-custom/toolchain-external-custom.mk
index c4ae6125ef..f2ec01f88c 100644
--- a/toolchain/toolchain-external/toolchain-external-custom/toolchain-external-custom.mk
+++ b/toolchain/toolchain-external/toolchain-external-custom/toolchain-external-custom.mk
@@ -10,6 +10,11 @@ TOOLCHAIN_EXTERNAL_CUSTOM_SOURCE = $(notdir $(call qstrip,$(BR2_TOOLCHAIN_EXTERN
 ifeq ($(BR2_TOOLCHAIN_EXTERNAL_CUSTOM),y)
 # We can't check hashes for custom downloaded toolchains
 BR_NO_CHECK_HASH_FOR += $(TOOLCHAIN_EXTERNAL_SOURCE)
+ifeq ($(BR_BUILDING)$(BR2_TOOLCHAIN_EXTERNAL_DOWNLOAD),yy)
+ifeq ($(TOOLCHAIN_EXTERNAL_CUSTOM_SOURCE),)
+$(error No external toolchain url set, check your BR2_TOOLCHAIN_EXTERNAL_URL setting)
+endif
+endif
 endif
 
 $(eval $(toolchain-external-package))