diff --git a/Makefile b/Makefile
index c483586..dd93938 100644
--- a/Makefile
+++ b/Makefile
@@ -73,7 +73,7 @@
 $(TARGET_DIR)/%.i5.xml: $(BUILD_DIR)/% xslt/epub2i5.xsl xslt/idsCorpus-template.xml
 	mkdir -p $(TARGET_DIR)
 	echo "Converting $< to $@"
-	$(SAXON) -xsl:xslt/epub2i5.xsl $(shell find $< -name "*.opf") > $@
+	$(SAXON) -xsl:xslt/epub2i5.xsl $(shell find $< -name "*.opf") > $@ || (echo "WARN: ignoring invalid $@" && > $@)
 
 %.zip: %.i5.xml
 	tei2korapxml -l warn -s -tk - < $< > $@