diff --git a/ultragrid/src/pdb.c b/ultragrid/src/pdb.c
index d62b596c5..943b8833c 100644
--- a/ultragrid/src/pdb.c
+++ b/ultragrid/src/pdb.c
@@ -95,7 +95,11 @@ struct pdb {
 #define BTREE_MAGIC      0x10101010
 #define BTREE_NODE_MAGIC 0x01010101
 
-static __thread int pdb_count;
+static
+#ifndef HAVE_MACOSX
+__thread
+#endif
+int pdb_count;
 
 static void pdb_validate_node(pdb_node_t * node, pdb_node_t * parent)
 {