diff --git a/src/ChangeLog b/src/ChangeLog
index fcbc7dcc3a9..43e56416e57 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -1,5 +1,9 @@
 2015-01-21  Paul Eggert  <eggert@cs.ucla.edu>
 
+	Fix coding.c subscript error
+	* coding.c (CODING_ISO_INVOKED_CHARSET):
+	Avoid undefined behavior if CODING_ISO_INVOCATION returns negative.
+
 	Backport: correct old fix for GTK font selection
 	* gtkutil.c (xg_get_font): Fix off-by-2 typo.
 	Fixes: bug#3228
diff --git a/src/coding.c b/src/coding.c
index 77583d0d950..8dc39e74422 100644
--- a/src/coding.c
+++ b/src/coding.c
@@ -395,7 +395,8 @@ static Lisp_Object Vbig5_coding_system;
 #define CODING_ISO_BOL(coding)	\
   ((coding)->spec.iso_2022.bol)
 #define CODING_ISO_INVOKED_CHARSET(coding, plane)	\
-  CODING_ISO_DESIGNATION ((coding), CODING_ISO_INVOCATION ((coding), (plane)))
+  (CODING_ISO_INVOCATION (coding, plane) < 0 ? -1	\
+   : CODING_ISO_DESIGNATION (coding, CODING_ISO_INVOCATION (coding, plane)))
 #define CODING_ISO_CMP_STATUS(coding)	\
   (&(coding)->spec.iso_2022.cmp_status)
 #define CODING_ISO_EXTSEGMENT_LEN(coding)	\