diff --git a/include/jemalloc/internal/emap.h b/include/jemalloc/internal/emap.h
index 9b925225..8b2c6ba0 100644
--- a/include/jemalloc/internal/emap.h
+++ b/include/jemalloc/internal/emap.h
@@ -127,6 +127,15 @@ emap_assert_mapped(tsdn_t *tsdn, emap_t *emap, edata_t *edata) {
 	}
 }
 
+/* Assert that the given edata isn't in the map. */
+void emap_do_assert_not_mapped(tsdn_t *tsdn, emap_t *emap, edata_t *edata);
+static inline void
+emap_assert_not_mapped(tsdn_t *tsdn, emap_t *emap, edata_t *edata) {
+	if (config_debug) {
+		emap_do_assert_not_mapped(tsdn, emap, edata);
+	}
+}
+
 JEMALLOC_ALWAYS_INLINE edata_t *
 emap_edata_lookup(tsdn_t *tsdn, emap_t *emap, const void *ptr) {
 	rtree_ctx_t rtree_ctx_fallback;
diff --git a/src/emap.c b/src/emap.c
index 4e7ca8d0..537f5884 100644
--- a/src/emap.c
+++ b/src/emap.c
@@ -323,3 +323,16 @@ emap_do_assert_mapped(tsdn_t *tsdn, emap_t *emap, edata_t *edata) {
 	assert(rtree_read(tsdn, &emap->rtree, rtree_ctx,
 	    (uintptr_t)edata_base_get(edata)).edata == edata);
 }
+
+void
+emap_do_assert_not_mapped(tsdn_t *tsdn, emap_t *emap, edata_t *edata) {
+	emap_full_alloc_ctx_t context1 = {0};
+	emap_full_alloc_ctx_try_lookup(tsdn, emap, edata_base_get(edata),
+	    &context1);
+	assert(context1.edata == NULL);
+
+	emap_full_alloc_ctx_t context2 = {0};
+	emap_full_alloc_ctx_try_lookup(tsdn, emap, edata_last_get(edata),
+	    &context2);
+	assert(context2.edata == NULL);
+}