diff --git a/doc/src/sgml/Makefile b/doc/src/sgml/Makefile
index 9c9bbfe375..f6d2c85226 100644
--- a/doc/src/sgml/Makefile
+++ b/doc/src/sgml/Makefile
@@ -194,7 +194,7 @@ MAKEINFO = makeinfo
 ##
 
 # Quick syntax check without style processing
-check: postgres.sgml $(ALLSGML) check-tabs
+check: postgres.sgml $(ALLSGML) check-tabs check-nbsp
 	$(XMLLINT) $(XMLINCLUDE) --noout --valid $<
 
 
@@ -259,6 +259,9 @@ endif # sqlmansectnum != 7
 check-tabs:
 	@( ! grep '	' $(wildcard $(srcdir)/*.sgml $(srcdir)/ref/*.sgml $(srcdir)/*.xsl) ) || (echo "Tabs appear in SGML/XML files" 1>&2;  exit 1)
 
+check-nbsp:
+	@( ! grep -e "\xA0" $(wildcard $(srcdir)/*.sgml $(srcdir)/ref/*.sgml $(srcdir)/*.xsl) ) || (echo "Non-breaking space appear in SGML/XML files" 1>&2;  exit 1)
+
 ##
 ## Clean
 ##