diff --git a/mysql-test/include/ctype_common.inc b/mysql-test/include/ctype_common.inc
index 202c508a9c9..9ee0a40c8ce 100644
--- a/mysql-test/include/ctype_common.inc
+++ b/mysql-test/include/ctype_common.inc
@@ -51,6 +51,15 @@ SELECT c1 as want1result  from t1 where c1 like 'locatio%';
 SELECT c1 as want1result  from t1 where c1 like 'location%';
 DROP TABLE t1;
 
+#
+# Bug #31070: crash during conversion of charsets
+#
+create table t1 (a set('a') not null);
+insert into t1 values (),();
+select cast(a as char(1)) from t1;
+select a sounds like a from t1;
+drop table t1;
+
 DROP DATABASE d1;
 # Restore settings
 USE test;
diff --git a/mysql-test/r/ctype_big5.result b/mysql-test/r/ctype_big5.result
index 6d318a445f5..b190273cc64 100644
--- a/mysql-test/r/ctype_big5.result
+++ b/mysql-test/r/ctype_big5.result
@@ -52,6 +52,19 @@ SELECT c1 as want1result  from t1 where c1 like 'location%';
 want1result
 location
 DROP TABLE t1;
+create table t1 (a set('a') not null);
+insert into t1 values (),();
+Warnings:
+Warning	1364	Field 'a' doesn't have a default value
+select cast(a as char(1)) from t1;
+cast(a as char(1))
+
+
+select a sounds like a from t1;
+a sounds like a
+1
+1
+drop table t1;
 DROP DATABASE d1;
 USE test;
 SET character_set_server= @safe_character_set_server;
diff --git a/mysql-test/r/ctype_euckr.result b/mysql-test/r/ctype_euckr.result
index 6017bc07763..b9619370d4c 100644
--- a/mysql-test/r/ctype_euckr.result
+++ b/mysql-test/r/ctype_euckr.result
@@ -52,6 +52,19 @@ SELECT c1 as want1result  from t1 where c1 like 'location%';
 want1result
 location
 DROP TABLE t1;
+create table t1 (a set('a') not null);
+insert into t1 values (),();
+Warnings:
+Warning	1364	Field 'a' doesn't have a default value
+select cast(a as char(1)) from t1;
+cast(a as char(1))
+
+
+select a sounds like a from t1;
+a sounds like a
+1
+1
+drop table t1;
 DROP DATABASE d1;
 USE test;
 SET character_set_server= @safe_character_set_server;
diff --git a/mysql-test/r/ctype_gb2312.result b/mysql-test/r/ctype_gb2312.result
index 314c336bab9..90c94c3b299 100644
--- a/mysql-test/r/ctype_gb2312.result
+++ b/mysql-test/r/ctype_gb2312.result
@@ -52,6 +52,19 @@ SELECT c1 as want1result  from t1 where c1 like 'location%';
 want1result
 location
 DROP TABLE t1;
+create table t1 (a set('a') not null);
+insert into t1 values (),();
+Warnings:
+Warning	1364	Field 'a' doesn't have a default value
+select cast(a as char(1)) from t1;
+cast(a as char(1))
+
+
+select a sounds like a from t1;
+a sounds like a
+1
+1
+drop table t1;
 DROP DATABASE d1;
 USE test;
 SET character_set_server= @safe_character_set_server;
diff --git a/mysql-test/r/ctype_gbk.result b/mysql-test/r/ctype_gbk.result
index 3f5d8b0d8c6..fe90c7bff29 100644
--- a/mysql-test/r/ctype_gbk.result
+++ b/mysql-test/r/ctype_gbk.result
@@ -52,6 +52,19 @@ SELECT c1 as want1result  from t1 where c1 like 'location%';
 want1result
 location
 DROP TABLE t1;
+create table t1 (a set('a') not null);
+insert into t1 values (),();
+Warnings:
+Warning	1364	Field 'a' doesn't have a default value
+select cast(a as char(1)) from t1;
+cast(a as char(1))
+
+
+select a sounds like a from t1;
+a sounds like a
+1
+1
+drop table t1;
 DROP DATABASE d1;
 USE test;
 SET character_set_server= @safe_character_set_server;
diff --git a/mysql-test/r/ctype_uca.result b/mysql-test/r/ctype_uca.result
index 889702e380c..e676d5a5ca0 100644
--- a/mysql-test/r/ctype_uca.result
+++ b/mysql-test/r/ctype_uca.result
@@ -2587,6 +2587,19 @@ SELECT c1 as want1result  from t1 where c1 like 'location%';
 want1result
 location
 DROP TABLE t1;
+create table t1 (a set('a') not null);
+insert into t1 values (),();
+Warnings:
+Warning	1364	Field 'a' doesn't have a default value
+select cast(a as char(1)) from t1;
+cast(a as char(1))
+
+
+select a sounds like a from t1;
+a sounds like a
+1
+1
+drop table t1;
 DROP DATABASE d1;
 USE test;
 SET character_set_server= @safe_character_set_server;
diff --git a/strings/ctype-big5.c b/strings/ctype-big5.c
index 7bb811a3064..ecfd3d648e0 100644
--- a/strings/ctype-big5.c
+++ b/strings/ctype-big5.c
@@ -6272,12 +6272,12 @@ my_mb_wc_big5(CHARSET_INFO *cs __attribute__((unused)),
 	      my_wc_t *pwc,const uchar *s,const uchar *e)
 {
 
-  int hi=s[0];
+  int hi;
   
   if (s >= e)
     return MY_CS_TOOSMALL;
   
-  if (hi<0x80)
+  if ((hi= s[0]) < 0x80)
   {
     pwc[0]=hi;
     return 1;
diff --git a/strings/ctype-euc_kr.c b/strings/ctype-euc_kr.c
index 844d8cc3ff5..fd783015bf4 100644
--- a/strings/ctype-euc_kr.c
+++ b/strings/ctype-euc_kr.c
@@ -8614,12 +8614,12 @@ my_mb_wc_euc_kr(CHARSET_INFO *cs __attribute__((unused)),
 		 my_wc_t *pwc, const uchar *s, const uchar *e)
 {
   
-  int hi=s[0];
+  int hi;
   
   if (s >= e)
     return MY_CS_TOOSMALL;
   
-  if (hi<0x80)
+  if ((hi= s[0]) < 0x80)
   {
     pwc[0]=hi;
     return 1;
diff --git a/strings/ctype-gb2312.c b/strings/ctype-gb2312.c
index 21aeb8b7990..4b3518ce3d2 100644
--- a/strings/ctype-gb2312.c
+++ b/strings/ctype-gb2312.c
@@ -5664,12 +5664,10 @@ my_mb_wc_gb2312(CHARSET_INFO *cs  __attribute__((unused)),
 		my_wc_t *pwc, const uchar *s, const uchar *e){
   int hi;
   
-  hi=(int) s[0];
-  
   if (s >= e)
     return MY_CS_TOOSMALL;
   
-  if (hi<0x80)
+  if ((hi= s[0]) < 0x80)
   {
     pwc[0]=hi;
     return 1;
diff --git a/strings/ctype-sjis.c b/strings/ctype-sjis.c
index ace457af63c..3925b76869c 100644
--- a/strings/ctype-sjis.c
+++ b/strings/ctype-sjis.c
@@ -4527,12 +4527,12 @@ mb:
 static int 
 my_mb_wc_sjis(CHARSET_INFO *cs  __attribute__((unused)),
 	      my_wc_t *pwc, const uchar *s, const uchar *e){
-  int hi=s[0];
+  int hi;
   
   if (s >= e)
     return MY_CS_TOOSMALL;
   
-  if (hi < 0x80)
+  if ((hi= s[0]) < 0x80)
   {
     pwc[0]=hi;
     return 1;