diff --git a/build/Makefile.vty-reference.inc b/build/Makefile.vty-reference.inc
index baf9cf5..6607ae5 100644
--- a/build/Makefile.vty-reference.inc
+++ b/build/Makefile.vty-reference.inc
@@ -26,7 +26,7 @@ DOCBOOKS_DEPS = generated/docbook_vty.xml
 include $(TOPDIR)/build/Makefile.docbook.inc
 
 LIBOSMO_DIR ?= ~/source/gsm/libosmocore
-MERGE_DOC = $(LIBOSMO_DIR)/doc/vty/merge_doc.xsl
+MERGE_DOC = $(shell realpath $(LIBOSMO_DIR)/doc/vty/merge_doc.xsl)
 
 CLEAN_FILES += generated