diff --git a/configure b/configure
index 9f7bded369..7a826cbd26 100755
--- a/configure
+++ b/configure
@@ -4818,11 +4818,19 @@ has_sphinx_build() {
 
 # Check if tools are available to build documentation.
 if test "$docs" != "no" ; then
-  if has makeinfo && has pod2man && has_sphinx_build; then
+  if has_sphinx_build; then
+    sphinx_ok=yes
+  else
+    sphinx_ok=no
+  fi
+  if has makeinfo && has pod2man && test "$sphinx_ok" = "yes"; then
     docs=yes
   else
     if test "$docs" = "yes" ; then
-      feature_not_found "docs" "Install texinfo, Perl/perl-podlators and python-sphinx"
+      if has $sphinx_build && test "$sphinx_ok" != "yes"; then
+        echo "Warning: $sphinx_build exists but it is either too old or uses too old a Python version" >&2
+      fi
+      feature_not_found "docs" "Install texinfo, Perl/perl-podlators and a Python 3 version of python-sphinx"
     fi
     docs=no
   fi
diff --git a/docs/conf.py b/docs/conf.py
index ee7faa6b4e..7588bf192e 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -28,6 +28,16 @@
 
 import os
 import sys
+import sphinx
+from sphinx.errors import VersionRequirementError
+
+# Make Sphinx fail cleanly if using an old Python, rather than obscurely
+# failing because some code in one of our extensions doesn't work there.
+# Unfortunately this doesn't display very neatly (there's an unavoidable
+# Python backtrace) but at least the information gets printed...
+if sys.version_info < (3,5):
+    raise VersionRequirementError(
+        "QEMU requires a Sphinx that uses Python 3.5 or better\n")
 
 # The per-manual conf.py will set qemu_docdir for a single-manual build;
 # otherwise set it here if this is an entire-manual-set build.