diff --git a/gfx/thebes/gfxSVGGlyphs.cpp b/gfx/thebes/gfxSVGGlyphs.cpp
index b5894c1e402..d2bad1d3495 100644
--- a/gfx/thebes/gfxSVGGlyphs.cpp
+++ b/gfx/thebes/gfxSVGGlyphs.cpp
@@ -118,7 +118,9 @@ gfxSVGGlyphsDocument* gfxSVGGlyphs::FindOrCreateGlyphsDocument(
     auto doc = MakeUnique<gfxSVGGlyphsDocument>(
         data + mHeader->mDocIndexOffset + entry->mDocOffset, entry->mDocLength,
         this);
-    return mGlyphDocs.InsertOrUpdate(entry->mDocOffset, std::move(doc)).get();
+    // Similarly to the Lookup() call above, if we've already created a separate
+    // document, keep it rather than overriding it.
+    return mGlyphDocs.LookupOrInsert(entry->mDocOffset, std::move(doc)).get();
   }
 
   return nullptr;