diff --git a/src/runtime/alloc.cpp b/src/runtime/alloc.cpp
index 5009e8b81a..72e3279079 100644
--- a/src/runtime/alloc.cpp
+++ b/src/runtime/alloc.cpp
@@ -347,6 +347,7 @@ static void finalize_heap(void * _h) {
 }
 
 static void init_heap(bool main) {
+    lean_assert(g_heap == nullptr);
     g_heap = new heap();
     g_heap->alloc_segment();
     unsigned obj_size = LEAN_OBJECT_SIZE_DELTA;
@@ -360,15 +361,19 @@ static void init_heap(bool main) {
         register_thread_finalizer(finalize_heap, g_heap);
 }
 
+void init_thread_heap() {
+    init_heap(false);
+}
+
 void * alloc(size_t sz) {
     sz = align(sz, LEAN_OBJECT_SIZE_DELTA);
     LEAN_RUNTIME_STAT_CODE(g_num_alloc++);
     if (LEAN_UNLIKELY(sz > LEAN_MAX_SMALL_OBJECT_SIZE)) {
-        return malloc(sz);
-    }
-    if (LEAN_UNLIKELY(g_heap == nullptr)) {
-        init_heap(false);
+        void * r = malloc(sz);
+        if (r == nullptr) throw std::bad_alloc();
+        return r;
     }
+    lean_assert(g_heap);
     LEAN_RUNTIME_STAT_CODE(g_num_small_alloc++);
     unsigned slot_idx = get_slot_idx(sz);
     page * p = g_heap->m_curr_page[slot_idx];
diff --git a/src/runtime/alloc.h b/src/runtime/alloc.h
index d0276535c1..467b0acda9 100644
--- a/src/runtime/alloc.h
+++ b/src/runtime/alloc.h
@@ -7,6 +7,7 @@ Author: Leonardo de Moura
 #include<cstddef>
 #pragma once
 namespace lean {
+void init_thread_heap();
 void * alloc(size_t sz);
 void dealloc(void * o, size_t sz);
 void initialize_alloc();
diff --git a/src/runtime/object.cpp b/src/runtime/object.cpp
index 69fa9b35e0..8dfbe39807 100644
--- a/src/runtime/object.cpp
+++ b/src/runtime/object.cpp
@@ -327,8 +327,8 @@ void * alloc_heap_object(size_t sz) {
     void * r = alloc(sizeof(rc_type) + sz);
 #else
     void * r = malloc(sizeof(rc_type) + sz);
-#endif
     if (r == nullptr) throw std::bad_alloc();
+#endif
     *static_cast<rc_type *>(r) = 1;
     return static_cast<char *>(r) + sizeof(rc_type);
 }
diff --git a/src/runtime/thread.cpp b/src/runtime/thread.cpp
index 1141f77367..1d55f95c30 100644
--- a/src/runtime/thread.cpp
+++ b/src/runtime/thread.cpp
@@ -15,6 +15,7 @@ Author: Leonardo de Moura
 #include "runtime/thread.h"
 #include "runtime/interrupt.h"
 #include "runtime/exception.h"
+#include "runtime/alloc.h"
 
 #ifndef LEAN_DEFAULT_THREAD_STACK_SIZE
 #define LEAN_DEFAULT_THREAD_STACK_SIZE 8*1024*1024 // 8Mb
@@ -44,6 +45,7 @@ void reset_thread_local() {
 using runnable = std::function<void()>;
 
 static void thread_main(void * p) {
+    init_thread_heap();
     std::unique_ptr<runnable> f;
     f.reset(reinterpret_cast<runnable *>(p));