--- a/dom/canvas/WebGLContext.cpp
+++ b/dom/canvas/WebGLContext.cpp
@@ -1868,16 +1868,24 @@ WebGLContext::DidRefresh()
 
 bool
 WebGLContext::ValidateCurFBForRead(const char* funcName,
                                    const webgl::FormatUsageInfo** const out_format,
                                    uint32_t* const out_width, uint32_t* const out_height,
                                    GLenum* const out_mode)
 {
     if (!mBoundReadFramebuffer) {
+        const GLenum readBufferMode = gl->Screen()->GetReadBufferMode();
+        if (readBufferMode == LOCAL_GL_NONE) {
+            ErrorInvalidOperation("%s: Can't read from backbuffer when readBuffer mode is"
+                                  " NONE.",
+                                  funcName);
+            return false;
+        }
+
         ClearBackbufferIfNeeded();
 
         // FIXME - here we're assuming that the default framebuffer is backed by
         // UNSIGNED_BYTE that might not always be true, say if we had a 16bpp default
         // framebuffer.
         auto effFormat = mOptions.alpha ? webgl::EffectiveFormat::RGBA8
                                         : webgl::EffectiveFormat::RGB8;