--- a/dom/canvas/WebGL2ContextFramebuffers.cpp
+++ b/dom/canvas/WebGL2ContextFramebuffers.cpp
@@ -311,16 +311,18 @@ WebGL2Context::InvalidateSubFramebuffer(
 
 void
 WebGL2Context::ReadBuffer(GLenum mode)
 {
     const char funcName[] = "readBuffer";
     if (IsContextLost())
         return;
 
+    gl->MakeCurrent();
+
     if (mBoundReadFramebuffer) {
         mBoundReadFramebuffer->ReadBuffer(funcName, mode);
         return;
     }
 
     // Operating on the default framebuffer.
     if (mode != LOCAL_GL_NONE &&
         mode != LOCAL_GL_BACK)