diff --git a/mysql-test/r/ps.result b/mysql-test/r/ps.result
index 566dac07fd5..cdba113ae40 100644
--- a/mysql-test/r/ps.result
+++ b/mysql-test/r/ps.result
@@ -622,3 +622,22 @@ execute stmt using @user_id, @id;
 partner_id
 deallocate prepare stmt;
 drop table t1, t2, t3, t4;
+prepare stmt from 'select ?=?';
+set @a='CHRISTINE           ';
+set @b='CHRISTINE';
+execute stmt using @a, @b;
+?=?
+1
+execute stmt using @a, @b;
+?=?
+1
+set @a=1, @b=2;
+execute stmt using @a, @b;
+?=?
+0
+set @a='CHRISTINE           ';
+set @b='CHRISTINE';
+execute stmt using @a, @b;
+?=?
+1
+deallocate prepare stmt;
diff --git a/mysql-test/t/ps.test b/mysql-test/t/ps.test
index a25d4447c20..0fa14024d11 100644
--- a/mysql-test/t/ps.test
+++ b/mysql-test/t/ps.test
@@ -650,3 +650,19 @@ execute stmt using @user_id, @id;
 execute stmt using @user_id, @id;
 deallocate prepare stmt;
 drop table t1, t2, t3, t4;
+
+#
+# Bug#9379: make sure that Item::collation is reset when one sets
+# a parameter marker from a string variable.
+#
+prepare stmt from 'select ?=?';
+set @a='CHRISTINE           ';
+set @b='CHRISTINE';
+execute stmt using @a, @b;
+execute stmt using @a, @b;
+set @a=1, @b=2;
+execute stmt using @a, @b;
+set @a='CHRISTINE           ';
+set @b='CHRISTINE';
+execute stmt using @a, @b;
+deallocate prepare stmt;
diff --git a/sql/item.cc b/sql/item.cc
index 19b88c115b9..ed39fe177cd 100644
--- a/sql/item.cc
+++ b/sql/item.cc
@@ -1089,6 +1089,7 @@ void Item_param::reset()
     to the binary log.
   */
   str_value.set_charset(&my_charset_bin);
+  collation.set(&my_charset_bin, DERIVATION_COERCIBLE);
   state= NO_VALUE;
   maybe_null= 1;
   null_value= 0;
@@ -1336,6 +1337,8 @@ bool Item_param::convert_str_value(THD *thd)
     */
     str_value_ptr.set(str_value.ptr(), str_value.length(),
                       str_value.charset());
+    /* Synchronize item charset with value charset */
+    collation.set(str_value.charset(), DERIVATION_COERCIBLE);
   }
   return rc;
 }
diff --git a/sql/item_func.cc b/sql/item_func.cc
index 442ef15bba8..4e0b6d8813f 100644
--- a/sql/item_func.cc
+++ b/sql/item_func.cc
@@ -188,7 +188,20 @@ bool Item_func::agg_arg_charsets(DTCollation &coll,
       break; // we cannot return here, we need to restore "arena".
     }
     conv->fix_fields(thd, 0, &conv);
-    *arg= conv;
+    /*
+      If in statement prepare, then we create a converter for two
+      constant items, do it once and then reuse it.
+      If we're in execution of a prepared statement, arena is NULL,
+      and the conv was created in runtime memory. This can be
+      the case only if the argument is a parameter marker ('?'),
+      because for all true constants the charset converter has already
+      been created in prepare. In this case register the change for
+      rollback.
+    */
+    if (arena)
+      *arg= conv;
+    else
+      thd->change_item_tree(arg, conv);
   }
   if (arena)
     thd->restore_backup_item_arena(arena, &backup);