diff --git a/Python/pystrtod.c b/Python/pystrtod.c
index e7bc22c..5a96b58 100644
--- a/Python/pystrtod.c
+++ b/Python/pystrtod.c
@@ -364,8 +364,8 @@
 	/* 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 _PyBytes_InsertThousandsGrouping(buffer, len, p,
-						 buf_size, NULL, 1);
+	return _PyBytes_InsertThousandsGrouping(buffer, len, p-buffer,
+						buf_size, NULL, 1);
 }
 
 /* see FORMATBUFLEN in unicodeobject.c */