diff --git a/mysql-test/r/heap.result b/mysql-test/r/heap.result
index 29bdfcbef7a..ddf675e2f73 100644
--- a/mysql-test/r/heap.result
+++ b/mysql-test/r/heap.result
@@ -731,3 +731,10 @@ SELECT COUNT(*) FROM t1 WHERE c=REPEAT('a',256);
 COUNT(*)
 2
 DROP TABLE t1;
+CREATE TABLE t1(c1 VARCHAR(100), c2 INT) ENGINE=MEMORY;
+INSERT INTO t1 VALUES('', 0);
+ALTER TABLE t1 MODIFY c1 VARCHAR(101);
+SELECT c2 FROM t1;
+c2
+0
+DROP TABLE t1;
diff --git a/mysql-test/t/heap.test b/mysql-test/t/heap.test
index 624597cd8d7..b47a5fc2033 100644
--- a/mysql-test/t/heap.test
+++ b/mysql-test/t/heap.test
@@ -471,3 +471,12 @@ SELECT COUNT(*) FROM t1 WHERE c=REPEAT('a',256);
 DROP TABLE t1;
 
 # End of 5.0 tests
+
+#
+# BUG#26080 - Memory Storage engine not working properly
+#
+CREATE TABLE t1(c1 VARCHAR(100), c2 INT) ENGINE=MEMORY;
+INSERT INTO t1 VALUES('', 0);
+ALTER TABLE t1 MODIFY c1 VARCHAR(101);
+SELECT c2 FROM t1;
+DROP TABLE t1;
diff --git a/storage/heap/ha_heap.cc b/storage/heap/ha_heap.cc
index cf11c9923eb..f2caa7e6d18 100644
--- a/storage/heap/ha_heap.cc
+++ b/storage/heap/ha_heap.cc
@@ -703,9 +703,10 @@ bool ha_heap::check_if_incompatible_data(HA_CREATE_INFO *info,
 					 uint table_changes)
 {
   /* Check that auto_increment value was not changed */
-  if ((table_changes != IS_EQUAL_YES &&
-       info->used_fields & HA_CREATE_USED_AUTO) &&
-      info->auto_increment_value != 0)
+  if ((info->used_fields & HA_CREATE_USED_AUTO &&
+       info->auto_increment_value != 0) ||
+      table_changes == IS_EQUAL_NO ||
+      table_changes & IS_EQUAL_PACK_LENGTH) // Not implemented yet
     return COMPATIBLE_DATA_NO;
   return COMPATIBLE_DATA_YES;
 }
diff --git a/storage/myisam/mi_open.c b/storage/myisam/mi_open.c
index 830332fe0c1..a06b21ee5b4 100644
--- a/storage/myisam/mi_open.c
+++ b/storage/myisam/mi_open.c
@@ -506,22 +506,6 @@ MI_INFO *mi_open(const char *name, int mode, uint open_flags)
       share->data_file_type = DYNAMIC_RECORD;
     my_afree((gptr) disk_cache);
     mi_setup_functions(share);
-    if (open_flags & HA_OPEN_MMAP)
-    {
-      info.s= share;
-      if (mi_dynmap_file(&info, share->state.state.data_file_length))
-      {
-        /* purecov: begin inspected */
-        /* Ignore if mmap fails. Use file I/O instead. */
-        DBUG_PRINT("warning", ("mmap failed: errno: %d", errno));
-        /* purecov: end */
-      }
-      else
-      {
-        share->file_read= mi_mmap_pread;
-        share->file_write= mi_mmap_pwrite;
-      }
-    }
     share->is_log_table= FALSE;
 #ifdef THREAD
     thr_lock_init(&share->lock);
@@ -552,6 +536,14 @@ MI_INFO *mi_open(const char *name, int mode, uint open_flags)
       }
     }
 #endif
+    /*
+      Memory mapping can only be requested after initializing intern_lock.
+    */
+    if (open_flags & HA_OPEN_MMAP)
+    {
+      info.s= share;
+      mi_extra(&info, HA_EXTRA_MMAP, 0);
+    }
   }
   else
   {