diff --git a/src/intervals.c b/src/intervals.c
index 0e3b684f570..f2a2a16f4ad 100644
--- a/src/intervals.c
+++ b/src/intervals.c
@@ -676,6 +676,7 @@ find_interval (register INTERVAL tree, register ptrdiff_t position)
 
   while (1)
     {
+      eassert (tree);
       if (relative_position < LEFT_TOTAL_LENGTH (tree))
 	{
 	  tree = tree->left;