--- a/gfx/layers/composite/CompositableHost.cpp
+++ b/gfx/layers/composite/CompositableHost.cpp
@@ -142,22 +142,33 @@ CompositableHost::SetCompositor(Composit
 
 bool
 CompositableHost::AddMaskEffect(EffectChain& aEffects,
                                 const gfx::Matrix4x4& aTransform,
                                 bool aIs3D)
 {
   RefPtr<TextureSource> source;
   RefPtr<TextureHost> host = GetAsTextureHost();
-  if (host && host->Lock()) {
-    source = host->GetTextureSources();
+
+  if (!host) {
+    NS_WARNING("Using compositable with no valid TextureHost as mask");
+    return false;
   }
 
+  if (!host->Lock()) {
+    NS_WARNING("Failed to lock the mask texture");
+    return false;
+  }
+
+  source = host->GetTextureSources();
+  MOZ_ASSERT(source);
+
   if (!source) {
-    NS_WARNING("Using compositable with no texture host as mask layer");
+    NS_WARNING("The TextureHost was successfully locked but can't provide a TextureSource");
+    host->Unlock();
     return false;
   }
 
   RefPtr<EffectMask> effect = new EffectMask(source,
                                              source->GetSize(),
                                              aTransform);
   effect->mIs3D = aIs3D;
   aEffects.mSecondaryEffects[EffectTypes::MASK] = effect;