diff --git a/src/lib/doctest.py b/src/lib/doctest.py
index 2ab49db972..4bcdd3b4c4 100644
--- a/src/lib/doctest.py
+++ b/src/lib/doctest.py
@@ -1 +1,9 @@
-raise NotImplementedError("doctest is not yet implemented in Skulpt")
+"""Stub implementation for Runestone Interactive books so students' doctests don't fail."""
+
+message = "The doctest module is not yet implemented by Skulpt. Please run your doctests in your own IDE!"
+
+def testmod():
+  print(message)
+
+if __name__ == "__main__":
+  print(message)