diff --git a/NEWS b/NEWS
index 4cbfe8128..93dfdd42e 100644
--- a/NEWS
+++ b/NEWS
@@ -34,6 +34,8 @@ New in spot 2.9.0.dev (not yet released)
 
   Bugs fixed:
 
+  - Work around undefined behavior reported by clang 10.0.0's UBsan.
+
   - spot::twa_sub_statistics was very slow at computing the number of
     transitons, and could overflow.  It is now avoiding some costly
     loop of BDD operations, and storing the result using at least 64
diff --git a/spot/misc/fixpool.cc b/spot/misc/fixpool.cc
index 30e0e45c3..06dbd55d8 100644
--- a/spot/misc/fixpool.cc
+++ b/spot/misc/fixpool.cc
@@ -1,5 +1,5 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2017-2018 Laboratoire de Recherche et
+// Copyright (C) 2017-2018, 2020 Laboratoire de Recherche et
 // Développement de l'Epita (LRDE)
 //
 // This file is part of Spot, a model checking library.
@@ -106,7 +106,20 @@ namespace spot
                 return (size + mask) & ~mask;
               }
           }(size)),
-      freelist_(nullptr), free_start_(nullptr),
-      free_end_(nullptr), chunklist_(nullptr)
-  {}
+      freelist_(nullptr),
+      chunklist_(nullptr)
+  {
+    new_chunk_();
+  }
+
+  void fixed_size_pool::new_chunk_()
+  {
+    const size_t requested = (size_ > 128 ? size_ : 128) * 8192 - 64;
+    chunk_* c = reinterpret_cast<chunk_*>(::operator new(requested));
+    c->prev = chunklist_;
+    chunklist_ = c;
+
+    free_start_ = c->data_ + size_;
+    free_end_ = c->data_ + requested;
+  }
 }
diff --git a/spot/misc/fixpool.hh b/spot/misc/fixpool.hh
index 4d79a192f..4d2068205 100644
--- a/spot/misc/fixpool.hh
+++ b/spot/misc/fixpool.hh
@@ -1,5 +1,5 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2011, 2015-2018 Laboratoire de Recherche et
+// Copyright (C) 2011, 2015-2018, 2020 Laboratoire de Recherche et
 // Développement de l'Epita (LRDE)
 //
 // This file is part of Spot, a model checking library.
@@ -68,15 +68,7 @@ namespace spot
 
       // If all the last chunk has been used, allocate one more.
       if (free_start_ + size_ > free_end_)
-        {
-          const size_t requested = (size_ > 128 ? size_ : 128) * 8192 - 64;
-          chunk_* c = reinterpret_cast<chunk_*>(::operator new(requested));
-          c->prev = chunklist_;
-          chunklist_ = c;
-
-          free_start_ = c->data_ + size_;
-          free_end_ = c->data_ + requested;
-        }
+        new_chunk_();
 
       void* res = free_start_;
       free_start_ += size_;
@@ -105,6 +97,8 @@ namespace spot
     }
 
   private:
+    void new_chunk_();
+
     const size_t size_;
     struct block_ { block_* next; }* freelist_;
     char* free_start_;