diff --git a/doc/Makefile.am b/doc/Makefile.am
index caa909d50d..aeabc0981d 100644
--- a/doc/Makefile.am
+++ b/doc/Makefile.am
@@ -9,6 +9,10 @@
 # Note the absence of the 'developer' directory here; development docs are
 # never built as part of a regular build. They are only built when explicitly
 # asked for. See comment further down.
+
+# Sphinx is not designed to be invoked multiple times against the same toctree.
+.NOTPARALLEL:
+
 SUBDIRS = manpages user
 AM_MAKEFLAGS = DESTDIR=${DESTDIR} infodir=${infodir} doczdir=${abs_srcdir}
 
diff --git a/doc/frr-sphinx.mk b/doc/frr-sphinx.mk
index f5aca7a655..3d91e8f63d 100644
--- a/doc/frr-sphinx.mk
+++ b/doc/frr-sphinx.mk
@@ -1,6 +1,9 @@
 # Makefile for Sphinx documentation
 #
 
+# Sphinx is not designed to be invoked multiple times against the same toctree.
+.NOTPARALLEL:
+
 # You can set these variables from the command line.
 SPHINXOPTS    ?=
 SPHINXBUILD   ?= sphinx-build