diff --git a/docs/reference/gio/Makefile.am b/docs/reference/gio/Makefile.am
index 4167069fd..64d95fe80 100644
--- a/docs/reference/gio/Makefile.am
+++ b/docs/reference/gio/Makefile.am
@@ -168,6 +168,7 @@ XSLTPROC_FLAGS = \
 
 endif
 
+CLEANFILES ?=
 CLEANFILES += $(man_MANS)
 
 EXTRA_DIST += $(man_MANS)
diff --git a/docs/reference/glib/Makefile.am b/docs/reference/glib/Makefile.am
index 0ca622eac..88404ed65 100644
--- a/docs/reference/glib/Makefile.am
+++ b/docs/reference/glib/Makefile.am
@@ -116,6 +116,7 @@ XSLTPROC_FLAGS = \
 
 endif
 
+CLEANFILES ?=
 CLEANFILES += $(man_MANS)
 
 EXTRA_DIST += $(man_MANS)
diff --git a/docs/reference/gobject/Makefile.am b/docs/reference/gobject/Makefile.am
index 051883a24..48d88f80c 100644
--- a/docs/reference/gobject/Makefile.am
+++ b/docs/reference/gobject/Makefile.am
@@ -92,6 +92,7 @@ XSLTPROC_FLAGS = \
 
 endif
 
+CLEANFILES ?=
 CLEANFILES += $(man_MANS)
 
 EXTRA_DIST += $(man_MANS)