diff --git a/config/sources/common.conf b/config/sources/common.conf
index b6d2f386de..4b4194dda4 100644
--- a/config/sources/common.conf
+++ b/config/sources/common.conf
@@ -55,6 +55,11 @@ function late_family_config__common_defaults_for_mainline_kernel() {
 	# KERNEL_MAJOR_MINOR is the key to all this stuff. It must be set.
 	if [[ -z ${KERNEL_MAJOR_MINOR} ]]; then
 		display_alert "KERNEL_MAJOR_MINOR is unset" "common_defaults_for_mainline" "warn"
+
+		# Done late (here) to allow family to set it first, so we don't have to add KERNEL_TARGET to all boards for experimental kernels.
+		if [[ "${BRANCH_VALID_FOR_BOARD}" == "no" ]]; then # Set in config-interactive by comparing KERNEL_TARGETS
+			exit_with_error "Invalid BRANCH='${BRANCH}' for BOARD='${BOARD}'; please select a valid branch for this board, eg one of '${KERNEL_TARGET}'"
+		fi
 		((problems++))
 	fi
 
diff --git a/lib/functions/main/config-interactive.sh b/lib/functions/main/config-interactive.sh
index 131dae5d94..37a1cd4db1 100644
--- a/lib/functions/main/config-interactive.sh
+++ b/lib/functions/main/config-interactive.sh
@@ -22,7 +22,13 @@ function config_possibly_interactive_kernel_board() {
 function config_possibly_interactive_branch_release_desktop_minimal() {
 	interactive_config_ask_branch
 	[[ -z $BRANCH ]] && exit_with_error "No kernel branch selected: BRANCH"
-	[[ ${KERNEL_TARGET} != *${BRANCH}* && ${BRANCH} != "ddk" ]] && exit_with_error "Kernel branch not defined for this board: '${BRANCH}' for '${BOARD}'"
+
+	# Check for BRANCH validity, warn but don't break the build if invalid; mark it as invalid for later checks -- if really no valid config, common.conf will exit with error later.
+	declare -g BRANCH_VALID_FOR_BOARD='yes'
+	if [[ ${KERNEL_TARGET} != *${BRANCH}* && ${BRANCH} != "ddk" ]]; then
+		display_alert "BRANCH not found for board" "BRANCH='${BRANCH}' not valid for BOARD='${BOARD}' - listed KERNEL_TARGET='${KERNEL_TARGET}'" "warn"
+		declare -g BRANCH_VALID_FOR_BOARD='no'
+	fi
 
 	interactive_config_ask_release
 	# If building image or rootfs (and thus "NEEDS_BINFMT=yes"), then RELEASE must be set.