diff --git a/lisp/progmodes/python.el b/lisp/progmodes/python.el
index f402ad83cca..50b9d1b0eee 100644
--- a/lisp/progmodes/python.el
+++ b/lisp/progmodes/python.el
@@ -4492,6 +4492,7 @@ point's current `syntax-ppss'."
                          (python-nav-backward-sexp)
                          (setq backward-sexp-point (point))
                          (and (= indentation (current-indentation))
+                              (not (bobp)) ; Prevent infloop.
                               (looking-at-p
                                (concat "[uU]?[rR]?"
                                        (python-rx string-delimiter)))))