diff --git a/include/mimalloc/prim.h b/include/mimalloc/prim.h
index 97d8b45d..68f0871e 100644
--- a/include/mimalloc/prim.h
+++ b/include/mimalloc/prim.h
@@ -104,12 +104,13 @@ void _mi_prim_thread_associate_default_heap(mi_heap_t* heap);
 
 
 //-------------------------------------------------------------------
-// Thread id
+// Thread id: `_mi_prim_thread_id()`
 // 
 // Getting the thread id should be performant as it is called in the
 // fast path of `_mi_free` and we specialize for various platforms as
 // inlined definitions. Regular code should call `init.c:_mi_thread_id()`.
-// We only require _mi_prim_thread_id() to return a unique id for each thread.
+// We only require _mi_prim_thread_id() to return a unique id
+// for each thread (unequal to zero).
 //-------------------------------------------------------------------
 
 static inline mi_threadid_t _mi_prim_thread_id(void) mi_attr_noexcept;
diff --git a/src/init.c b/src/init.c
index b402a3df..fcfcb659 100644
--- a/src/init.c
+++ b/src/init.c
@@ -409,15 +409,24 @@ void mi_thread_done(void) mi_attr_noexcept {
   _mi_thread_done(NULL);
 }
 
+#include <unistd.h>
+
 void _mi_thread_done(mi_heap_t* heap) 
 {
-  mi_atomic_decrement_relaxed(&thread_count);
-  _mi_stat_decrease(&_mi_stats_main.threads, 1);
-
+  // calling with NULL implies using the default heap
   if (heap == NULL) { 
     heap = mi_prim_get_default_heap(); 
     if (heap == NULL) return;
   }
+
+  // prevent re-entrancy through heap_done/heap_set_default_direct (issue #699)
+  if (!mi_heap_is_initialized(heap)) {
+    return; 
+  }
+
+  // adjust stats
+  mi_atomic_decrement_relaxed(&thread_count);
+  _mi_stat_decrease(&_mi_stats_main.threads, 1);
   
   // check thread-id as on Windows shutdown with FLS the main (exit) thread may call this on thread-local heaps...
   if (heap->thread_id != _mi_thread_id()) return;
diff --git a/src/prim/unix/prim.c b/src/prim/unix/prim.c
index 0ac69f1a..ac23e5ca 100644
--- a/src/prim/unix/prim.c
+++ b/src/prim/unix/prim.c
@@ -629,8 +629,8 @@ bool _mi_prim_getenv(const char* name, char* result, size_t result_size) {
   if (len == 0) return false;
   char** env = mi_get_environ();
   if (env == NULL) return false;
-  // compare up to 256 entries
-  for (int i = 0; i < 256 && env[i] != NULL; i++) {
+  // compare up to 64K entries
+  for (int i = 0; i < 64*MI_KiB && env[i] != NULL; i++) {
     const char* s = env[i];
     if (_mi_strnicmp(name, s, len) == 0 && s[len] == '=') { // case insensitive
       // found it
diff --git a/test/main-override.cpp b/test/main-override.cpp
index 65df79f1..18ec610c 100644
--- a/test/main-override.cpp
+++ b/test/main-override.cpp
@@ -39,12 +39,16 @@ static void strdup_test();            // issue #445
 static void bench_alloc_large(void);  // issue #xxx
 static void test_large_migrate(void); // issue #691
 static void heap_thread_free_huge();
+static void test_std_string();        // issue #697
 
 static void test_stl_allocators();
 
 
 int main() {
   mi_stats_reset();  // ignore earlier allocations  
+  
+  // test_std_string();
+  // heap_thread_free_huge();
   /*
    heap_thread_free_huge();
    heap_thread_free_large();
@@ -237,6 +241,13 @@ static void heap_no_delete() {
 }
 
 
+// Issue #697
+static void test_std_string() {
+  std::string path = "/Users/xxxx/Library/Developer/Xcode/DerivedData/xxxxxxxxxx/Build/Intermediates.noindex/xxxxxxxxxxx/arm64/XX_lto.o/0.arm64.lto.o";
+  std::string path1 = "/Users/xxxx/Library/Developer/Xcode/DerivedData/xxxxxxxxxx/Build/Intermediates.noindex/xxxxxxxxxxx/arm64/XX_lto.o/1.arm64.lto.o";
+  std::cout << path + "\n>>>            " + path1 + "\n>>>            " << std::endl;
+}
+
 // Issue #204
 static volatile void* global_p;