diff --git a/doc/Makefile.am b/doc/Makefile.am
index ca13bcf2d..3a2c1c2eb 100644
--- a/doc/Makefile.am
+++ b/doc/Makefile.am
@@ -182,8 +182,8 @@ defsincdate: $(gnupg_TEXINFOS)
 	if test -e $(top_srcdir)/.git; then \
 	  (cd $(srcdir) && git log -1 --format='%ct' \
                -- $(gnupg_TEXINFOS) 2>/dev/null) >>defsincdate; \
-        elif test x"$SOURCE_DATE_EPOCH" != x; then   \
-	   echo "$SOURCE_DATE_EPOCH" >>defsincdate ; \
+        elif test x"$$SOURCE_DATE_EPOCH" != x; then   \
+	   echo "$$SOURCE_DATE_EPOCH" >>defsincdate ; \
 	fi
 
 defs.inc : defsincdate Makefile mkdefsinc