diff --git a/make_doc.sh b/make_doc.sh
index b1441e26..c2e11ab4 100644
--- a/make_doc.sh
+++ b/make_doc.sh
@@ -12,7 +12,7 @@ set -euo pipefail
 # The Python interpreter can be overridden with $PYTHON (default: python).
 
 OUT="${1:-public}"
-PYTHON="${PYTHON:-python}"
+PYTHON="${PYTHON:-python3}"
 
 # Resolve the output directory to an absolute path before we change directories.
 mkdir -p "$OUT"