diff --git a/src/backend/mmap.c b/src/backend/mmap.c
index 0ff0c8d61..6622d8466 100644
--- a/src/backend/mmap.c
+++ b/src/backend/mmap.c
@@ -192,7 +192,7 @@ lumiera_mmap_init (LumieraMMap self, LumieraFile file, off_t start, size_t size)
   self->size = length;
   self->address = addr;
   self->refmap = lumiera_calloc ((length-1)/chunksize+1, sizeof (unsigned short));
-  self->refcnt = 0;
+  self->refcnt = 1;
   lumiera_mmapcache_announce (self);
 
   lumiera_file_handle_release (file);
@@ -228,7 +228,7 @@ lumiera_mmap_delete (LumieraMMap self)
   TRACE (mmap_dbg);
   if (self)
     {
-      REQUIRE (!self->refcnt);
+      REQUIRE (self->refcnt <= 1);
       lumiera_mmapcache_forget (self);
 
       /* The matching mappings->lock must be hold or being irrelevant (mappings destructor) here, we can't asset this from here, good luck */
diff --git a/src/backend/mmapings.c b/src/backend/mmapings.c
index b27801b70..0d8dc9051 100644
--- a/src/backend/mmapings.c
+++ b/src/backend/mmapings.c
@@ -59,8 +59,9 @@ lumiera_mmapings_destroy (LumieraMMapings self)
 
   LLIST_WHILE_TAIL (&self->mmaps, node)
     {
-      LumieraMMap mmap = LLIST_TO_STRUCTP (node, lumiera_mmap, searchnode);
-      lumiera_mmap_delete (mmap);
+      LumieraMMap map = LLIST_TO_STRUCTP (node, lumiera_mmap, searchnode);
+      REQUIRE (map->refcnt == 0, "map still in use: %p", map);
+      lumiera_mmap_delete (map);
     }
 
   lumiera_mutex_destroy (&self->lock, &NOBUG_FLAG(mutex_dbg), NOBUG_CONTEXT);
@@ -114,8 +115,9 @@ lumiera_mmapings_mmap_acquire (LumieraMMapings self, LumieraFile file, off_t sta
         if (ret)
           {
             if (!ret->refcnt)
-              /* in cache, needs to me checked out */
+              /* in cache, needs to be checked out */
               lumiera_mmapcache_checkout (ret);
+            ++ret->refcnt;
           }
         else
           {
@@ -128,8 +130,6 @@ lumiera_mmapings_mmap_acquire (LumieraMMapings self, LumieraFile file, off_t sta
             TODO ("sort search list?");
           }
 
-        ++ret->refcnt;
-
         PLANNED ("use refmap for finer grained refcounting");
 
         ENSURE (llist_is_empty(&ret->cachenode));