diff --git a/mysys/mf_dirname.c b/mysys/mf_dirname.c
index 378fe7080b8..43fde95554a 100644
--- a/mysys/mf_dirname.c
+++ b/mysys/mf_dirname.c
@@ -72,7 +72,9 @@ uint dirname_part(my_string to, const char *name)
 
   SYNPOSIS
     convert_dirname()
-    to				Store result here
+    to				Store result here. Must be at least of size
+    				min(FN_REFLEN, strlen(from) + 1) to make room
+    				for adding FN_LIBCHAR at the end.
     from			Original filename. May be == to
     from_end			Pointer at end of filename (normally end \0)
 
diff --git a/server-tools/instance-manager/instance_options.cc b/server-tools/instance-manager/instance_options.cc
index b05e40734b7..c10819b51b8 100644
--- a/server-tools/instance-manager/instance_options.cc
+++ b/server-tools/instance-manager/instance_options.cc
@@ -420,8 +420,13 @@ int Instance_options::complete_initialization(const char *default_path)
   const char *tmp;
   char *end;
 
-  if (!mysqld_path.str && !(mysqld_path.str= strdup_root(&alloc, default_path)))
-    goto err;
+  if (!mysqld_path.str)
+  {
+    // Need one extra byte, as convert_dirname() adds a slash at the end.
+    if (!(mysqld_path.str= alloc_root(&alloc, strlen(default_path) + 2)))
+      goto err;
+    strcpy(mysqld_path.str, default_path);
+  }
 
   // it's safe to cast this to char* since this is a buffer we are allocating
   end= convert_dirname((char*)mysqld_path.str, mysqld_path.str, NullS);