diff --git a/dom/media/gmp/GMPSharedMemManager.cpp b/dom/media/gmp/GMPSharedMemManager.cpp
index 06e4fd241ce..86b5f981032 100644
--- a/dom/media/gmp/GMPSharedMemManager.cpp
+++ b/dom/media/gmp/GMPSharedMemManager.cpp
@@ -40,9 +40,9 @@ GMPSharedMemManager::MgrAllocShmem(GMPSharedMem::GMPMemoryClasses aClass, size_t
   size_t pagesize = ipc::SharedMemory::SystemPageSize();
   aSize = (aSize + (pagesize-1)) & ~(pagesize-1); // round up to page size
   bool retval = Alloc(aSize, aType, aMem);
-  // The allocator (or NeedsShmem call) should never return less than we ask for...
-  MOZ_ASSERT(aMem->Size<uint8_t>() >= aSize);
   if (retval) {
+    // The allocator (or NeedsShmem call) should never return less than we ask for...
+    MOZ_ASSERT(aMem->Size<uint8_t>() >= aSize);
     mData->mGmpAllocated[aClass]++;
   }
   return retval;