diff --git a/bytestring-tree-builder.cabal b/bytestring-tree-builder.cabal
--- a/bytestring-tree-builder.cabal
+++ b/bytestring-tree-builder.cabal
@@ -1,7 +1,7 @@
 name:
   bytestring-tree-builder
 version:
-  0.2.1.3
+  0.2.1.4
 category:
   ByteString
 synopsis:
diff --git a/library/ByteString/TreeBuilder/Tree.hs b/library/ByteString/TreeBuilder/Tree.hs
--- a/library/ByteString/TreeBuilder/Tree.hs
+++ b/library/ByteString/TreeBuilder/Tree.hs
@@ -41,7 +41,7 @@
 foldlM step init =
   \case
     Empty ->
-      pure init
+      return init
     Leaf value ->
       step init value
     Branch tree1 tree2 ->