diff --git a/doc/Makefile.am b/doc/Makefile.am
index b0ccf234962..10e6f2f81f1 100644
--- a/doc/Makefile.am
+++ b/doc/Makefile.am
@@ -44,7 +44,11 @@ endif # BUILD_MANPAGES
 # see https://media.readthedocs.org/pdf/sphinx/1.6.3/sphinx.pdf
 # section 24.3.2 around page 247, third item for 'NotImplementedError', so this is kind of useless.
 
-ALLSPHINXOPTS   = $(SPHINXOPTS) -W
+# Building with errors as warnings is fragile. What works for English may not
+# when translated into Japanese, for example. To address this, we have the
+# default configuration be to treat warnings as errors, but give up if the user
+# passes SPHINXOPTS on the command line.
+ALLSPHINXOPTS   = $(or $(SPHINXOPTS),-W)
 # the i18n builder cannot share the environment and doctrees with the others
 I18NSPHINXOPTS  = $(SPHINXOPTS)