diff --git a/image/SurfaceCache.cpp b/image/SurfaceCache.cpp
index 26cb5c637742..22f787d3351d 100644
--- a/image/SurfaceCache.cpp
+++ b/image/SurfaceCache.cpp
@@ -1622,7 +1622,9 @@ void SurfaceCache::Initialize() {
   // Compute the size of the surface cache.
   uint64_t memorySize = PR_GetPhysicalMemorySize();
   if (memorySize == 0) {
+#if !defined(__DragonFly__)
     MOZ_ASSERT_UNREACHABLE("PR_GetPhysicalMemorySize not implemented here");
+#endif
     memorySize = 256 * 1024 * 1024;  // Fall back to 256MB.
   }
   uint64_t proposedSize = memorySize / surfaceCacheSizeFactor;