diff --git a/module/language/cps/intmap.scm b/module/language/cps/intmap.scm
index 7be27c906..152985a06 100644
--- a/module/language/cps/intmap.scm
+++ b/module/language/cps/intmap.scm
@@ -121,6 +121,9 @@
   (match bs
     (($ <intmap> min shift root)
      (cond
+      ((< i 0)
+       ;; The power-of-two spanning trick doesn't work across 0.
+       (error "Intmaps can only map non-negative integers." i))
       ((not val) (intmap-remove bs i))
       ((not root)
        ;; Add first element.
diff --git a/module/language/cps/intset.scm b/module/language/cps/intset.scm
index b235c61b0..1ec6119cd 100644
--- a/module/language/cps/intset.scm
+++ b/module/language/cps/intset.scm
@@ -152,6 +152,9 @@
   (match bs
     (($ <intset> min shift root)
      (cond
+      ((< i 0)
+       ;; The power-of-two spanning trick doesn't work across 0.
+       (error "Intsets can only hold non-negative integers." i))
       ((not root)
        ;; Add first element.
        (let ((min (round-down i shift)))