gistfile1.txt

diff --git a/src/gc/allocation.c b/src/gc/allocation.c
index 2d31dc6..ebb300e 100644
--- a/src/gc/allocation.c
+++ b/src/gc/allocation.c
@@ -104,5 +104,7 @@ void MVM_gc_allocate_gen2_default_set(MVMThreadContext *tc) {
 
 /* Sets allocation for this thread to be from the nursery by default. */
 void MVM_gc_allocate_gen2_default_clear(MVMThreadContext *tc) {
+    if (tc->allocate_in_gen2 <= 0)
+        MVM_oops(tc, "Cannot leave gen2 allocation without entering it");
     tc->allocate_in_gen2--;
 }