The model assertions for 18 don't actually use series=18 because that would be too logical.

live-build/auto/config

@@ -172,15 +172,17 @@ case $IMAGEFORMAT in
 		esac
 		case $SUITE in
 			xenial)
-				CORE_SERIES=16 ;;
+				# Ubuntu Core 16
+				;;
 			*)
-				CORE_SERIES=18 ;;
+				# Ubuntu Core 18
+				MODEL="ubuntu-core-18-$MODEL" ;;
 		esac
 
 		echo "IMAGEFORMAT=$IMAGEFORMAT" >> config/common
 		echo "UBUNTU_IMAGE_ARGS=\"$UBUNTU_IMAGE_ARGS\"" >> config/common
 		# Store model assertion in top dir to get it picked up later as a build artifact
-		env SNAPPY_STORE_NO_CDN=1 snap known --remote model series=$CORE_SERIES model="$MODEL" brand-id=canonical > "$PREFIX".model-assertion
+		env SNAPPY_STORE_NO_CDN=1 snap known --remote model series=16 model="$MODEL" brand-id=canonical > "$PREFIX".model-assertion
 		echo "Configured ubuntu-image for the following model assertion:"
 		cat "$PREFIX".model-assertion
 		echo "----------------------------------------------------------"