diff --git a/tests/buffer_tests.cpp b/tests/buffer_tests.cpp
index a5a0c2a..7563448 100644
--- a/tests/buffer_tests.cpp
+++ b/tests/buffer_tests.cpp
@@ -227,6 +227,18 @@
   }
 }
 
+// Malloc can return a tagged pointer, which is not accepted in mm system calls like mprotect.
+// Clear top 8 bits of the address on 64-bit platforms.
+static int MprotectHeap(void* addr, size_t len, int prot) {
+#if defined(__LP64__)
+  constexpr uintptr_t mask = (static_cast<uintptr_t>(1) << 56) - 1;
+  void* untagged_addr = reinterpret_cast<void*>(reinterpret_cast<uintptr_t>(addr) & mask);
+#else
+  void* untagged_addr = addr;
+#endif
+  return mprotect(untagged_addr, len, prot);
+}
+
 void RunSingleBufferAlignTest(
     size_t max_test_size, void (*test_func)(uint8_t*, size_t),
     size_t (*set_incr)(size_t)) {
@@ -358,14 +370,14 @@
   memset(memory, 0x23, 2*pagesize);
 
   // Make the second page unreadable and unwritable.
-  ASSERT_TRUE(mprotect(&memory[pagesize], pagesize, PROT_NONE) == 0);
+  ASSERT_TRUE(MprotectHeap(&memory[pagesize], pagesize, PROT_NONE) == 0);
 
   for (size_t i = 0; i < pagesize; i++) {
     uint8_t* buf = &memory[pagesize-i];
 
     test_func(buf, i);
   }
-  ASSERT_TRUE(mprotect(&memory[pagesize], pagesize, PROT_READ | PROT_WRITE) == 0);
+  ASSERT_TRUE(MprotectHeap(&memory[pagesize], pagesize, PROT_READ | PROT_WRITE) == 0);
   free(memory);
 }
 
@@ -379,7 +391,7 @@
   memset(memory, 0x23, 2*pagesize);
 
   // Make the second page unreadable and unwritable.
-  ASSERT_TRUE(mprotect(&memory[pagesize], pagesize, PROT_NONE) == 0);
+  ASSERT_TRUE(MprotectHeap(&memory[pagesize], pagesize, PROT_NONE) == 0);
 
   uint8_t* dst_buffer = new uint8_t[2*pagesize];
   // Change the dst alignment as we change the source.
@@ -391,7 +403,7 @@
       test_func(src, dst, j);
     }
   }
-  ASSERT_TRUE(mprotect(&memory[pagesize], pagesize, PROT_READ | PROT_WRITE) == 0);
+  ASSERT_TRUE(MprotectHeap(&memory[pagesize], pagesize, PROT_READ | PROT_WRITE) == 0);
   free(memory);
   delete[] dst_buffer;
 }
@@ -409,7 +421,7 @@
   memset(memory1, 0x23, 2*pagesize);
 
   // Make the second page unreadable and unwritable.
-  ASSERT_TRUE(mprotect(&memory1[pagesize], pagesize, PROT_NONE) == 0);
+  ASSERT_TRUE(MprotectHeap(&memory1[pagesize], pagesize, PROT_NONE) == 0);
 
   uint8_t* memory2;
   ASSERT_TRUE(posix_memalign(reinterpret_cast<void**>(&memory2), pagesize,
@@ -417,7 +429,7 @@
   memset(memory2, 0x23, 2*pagesize);
 
   // Make the second page unreadable and unwritable.
-  ASSERT_TRUE(mprotect(&memory2[pagesize], pagesize, PROT_NONE) == 0);
+  ASSERT_TRUE(MprotectHeap(&memory2[pagesize], pagesize, PROT_NONE) == 0);
 
   for (size_t i = 0; i < pagesize; i++) {
     uint8_t* buf1 = &memory1[pagesize-i];
@@ -445,8 +457,8 @@
     }
   }
 
-  ASSERT_TRUE(mprotect(&memory1[pagesize], pagesize, PROT_READ | PROT_WRITE) == 0);
-  ASSERT_TRUE(mprotect(&memory2[pagesize], pagesize, PROT_READ | PROT_WRITE) == 0);
+  ASSERT_TRUE(MprotectHeap(&memory1[pagesize], pagesize, PROT_READ | PROT_WRITE) == 0);
+  ASSERT_TRUE(MprotectHeap(&memory2[pagesize], pagesize, PROT_READ | PROT_WRITE) == 0);
   free(memory1);
   free(memory2);
 }