diff --git a/src/library/blast/blast.cpp b/src/library/blast/blast.cpp
index f4da6f99c..a097410d9 100644
--- a/src/library/blast/blast.cpp
+++ b/src/library/blast/blast.cpp
@@ -443,6 +443,8 @@ public:
     tmp_type_context * mk_tmp_type_context();
 
     void recycle_tmp_type_context(tmp_type_context * ctx) {
+        lean_assert(ctx);
+        ctx->clear();
         m_tmp_ctx_pool.push_back(ctx);
     }
 };