--- a/dom/canvas/WebGLContextGL.cpp
+++ b/dom/canvas/WebGLContextGL.cpp
@@ -1613,16 +1613,21 @@ ValidateReadPixelsFormatAndType(const we
     case LOCAL_GL_LUMINANCE:
     case LOCAL_GL_LUMINANCE_ALPHA:
     case LOCAL_GL_DEPTH_COMPONENT:
     case LOCAL_GL_DEPTH_STENCIL:
         webgl->ErrorInvalidEnum("readPixels: Invalid format: 0x%04x", pi.format);
         return false;
     }
 
+    if (pi.type == LOCAL_GL_UNSIGNED_INT_24_8) {
+        webgl->ErrorInvalidEnum("readPixels: Invalid type: 0x%04x", pi.type);
+        return false;
+    }
+
     MOZ_ASSERT(gl->IsCurrent());
     if (gl->IsSupported(gl::GLFeature::ES2_compatibility)) {
         const auto auxFormat = gl->GetIntAs<GLenum>(LOCAL_GL_IMPLEMENTATION_COLOR_READ_FORMAT);
         const auto auxType = gl->GetIntAs<GLenum>(LOCAL_GL_IMPLEMENTATION_COLOR_READ_TYPE);
 
         if (auxFormat && auxType &&
             pi.format == auxFormat && pi.type == auxType)
         {