diff --git a/tests/test_generated_code.c b/tests/test_generated_code.c
index 1de7673fbf..9cd5db7a5b 100644
--- a/tests/test_generated_code.c
+++ b/tests/test_generated_code.c
@@ -8,6 +8,7 @@
 #include "tests/test.upb.h"
 
 #define MIN(x, y) ((x) < (y) ? (x) : (y))
+#define UNUSED(x) (void)x
 
 const char test_str[] = "abcdefg";
 const char test_str2[] = "12345678910";
@@ -444,20 +445,32 @@ void test_arena_fuse(void) {
   ASSERT(i4 == 4);
 }
 
+/* Do nothing allocator for testing */
+static void *test_allocfunc(upb_alloc *alloc, void *ptr, size_t oldsize,
+                            size_t size) {
+  return upb_alloc_global.func(alloc, ptr, oldsize, size);
+}
+upb_alloc test_alloc = {&test_allocfunc};
+
 void test_arena_fuse_with_initial_block(void) {
-  char buf1[4096];
-  char buf2[4096];
-  upb_arena *arena1 = upb_arena_init(buf1, 4096, &upb_alloc_global);
-  upb_arena *arena2 = upb_arena_init(buf2, 4096, &upb_alloc_global);
-  upb_arena *arena3 = upb_arena_init(NULL, 0, &upb_alloc_global);
-  ASSERT(upb_arena_fuse(arena1, arena1));
-  ASSERT(!upb_arena_fuse(arena1, arena2));
-  ASSERT(!upb_arena_fuse(arena1, arena3));
-  ASSERT(!upb_arena_fuse(arena3, arena2));
+  char buf1[1024];
+  char buf2[1024];
+  upb_arena *arenas[] = {upb_arena_init(buf1, 1024, &upb_alloc_global),
+                         upb_arena_init(buf2, 1024, &upb_alloc_global),
+                         upb_arena_init(NULL, 0, &test_alloc),
+                         upb_arena_init(NULL, 0, &upb_alloc_global)};
+  int size = sizeof(arenas)/sizeof(arenas[0]);
+  for (int i = 0; i < size; ++i) {
+    for (int j = 0; j < size; ++j) {
+      if (i == j) {
+        ASSERT(upb_arena_fuse(arenas[i], arenas[j]));
+      } else {
+        ASSERT(!upb_arena_fuse(arenas[i], arenas[j]));
+      }
+    }
+  }
 
-  upb_arena_free(arena1);
-  upb_arena_free(arena2);
-  upb_arena_free(arena3);
+  for (int i = 0; i < size; ++i) upb_arena_free(arenas[i]);
 }
 
 void test_arena_decode(void) {
diff --git a/upb/upb.c b/upb/upb.c
index 5ed7eb396c..e5e16922cd 100644
--- a/upb/upb.c
+++ b/upb/upb.c
@@ -68,14 +68,15 @@ static void *upb_global_allocfunc(upb_alloc *alloc, void *ptr, size_t oldsize,
 }
 
 static uint32_t *upb_cleanup_pointer(uintptr_t cleanup_metadata) {
-  return (uint32_t*)(cleanup_metadata & ~0x1);
+  return (uint32_t *)(cleanup_metadata & ~0x1);
 }
 
 static bool upb_cleanup_has_initial_block(uintptr_t cleanup_metadata) {
   return cleanup_metadata & 0x1;
 }
 
-static uintptr_t upb_cleanup_metadata(uint32_t* cleanup, bool has_initial_block) {
+static uintptr_t upb_cleanup_metadata(uint32_t *cleanup,
+                                      bool has_initial_block) {
   return (uintptr_t)cleanup | has_initial_block;
 }
 
@@ -260,9 +261,14 @@ bool upb_arena_fuse(upb_arena *a1, upb_arena *a2) {
   upb_arena *r2 = arena_findroot(a2);
 
   if (r1 == r2) return true;  /* Already fused. */
+
+  /* Do not fuse initial blocks since we cannot lifetime extend them. */
   if (upb_cleanup_has_initial_block(r1->cleanup_metadata)) return false;
   if (upb_cleanup_has_initial_block(r2->cleanup_metadata)) return false;
 
+  /* Only allow fuse with a common allocator */
+  if (r1->block_alloc != r2->block_alloc) return false;
+
   /* We want to join the smaller tree to the larger tree.
    * So swap first if they are backwards. */
   if (r1->refcount < r2->refcount) {