diff --git a/.gitmodules b/.gitmodules
index 43b424fd9..f39a21a09 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -5,3 +5,6 @@
 [submodule "ext/phreeqcrm"]
 	path = ext/phreeqcrm
 	url = ../../mluebke/phreeqcrm-gfz.git
+[submodule "ext/doctest"]
+	path = ext/doctest
+	url = https://github.com/doctest/doctest.git
diff --git a/ext/doctest b/ext/doctest
new file mode 160000
index 000000000..8fdfd113d
--- /dev/null
+++ b/ext/doctest
@@ -0,0 +1 @@
+Subproject commit 8fdfd113dcb4ad1a31705ff8ddb13a21a505bad8