--- a/dom/canvas/WebGLContextDraw.cpp
+++ b/dom/canvas/WebGLContextDraw.cpp
@@ -624,17 +624,17 @@ WebGLContext::DrawElements_check(const c
 
     ////
 
     if (IsWebGL2() && !gl->IsSupported(gl::GLFeature::prim_restart_fixed)) {
         MOZ_ASSERT(gl->IsSupported(gl::GLFeature::prim_restart));
         if (mPrimRestartTypeBytes != bytesPerElem) {
             mPrimRestartTypeBytes = bytesPerElem;
 
-            const uint32_t ones = UINT32_MAX >> (4 - mPrimRestartTypeBytes);
+            const uint32_t ones = UINT32_MAX >> (32 - 8*mPrimRestartTypeBytes);
             gl->fEnable(LOCAL_GL_PRIMITIVE_RESTART);
             gl->fPrimitiveRestartIndex(ones);
         }
     }
 
     ////
 
     const GLsizei first = byteOffset / bytesPerElem;