diff --git a/Python/pystrtod.c b/Python/pystrtod.c
index 3f0328e..01c1c42 100644
--- a/Python/pystrtod.c
+++ b/Python/pystrtod.c
@@ -364,7 +364,7 @@
 	/* At this point, p points just past the right-most character we
 	   want to format.  We need to add the grouping string for the
 	   characters between buffer and p. */
-	return _PyString_InsertThousandsGrouping(buffer, len, p,
+	return _PyString_InsertThousandsGrouping(buffer, len, p-buffer,
 						 buf_size, NULL, 1);
 }