--- a/layout/style/nsCSSDataBlock.cpp
+++ b/layout/style/nsCSSDataBlock.cpp
@@ -66,20 +66,22 @@ TryToStartImageLoadOnValue(const nsCSSVa
     // For example,
     //   mask-image: url(#mask_id); // refer to a SVG mask element, whose id is
     //                              // "mask_id", in the current document.
     // For such 'mask-image' values (pointing to an in-document element),
     // there is no need to trigger image download.
     if (aProperty == eCSSProperty_mask_image) {
       nsIURI* docURI = aDocument->GetDocumentURI();
       nsIURI* imageURI = aValue.GetURLValue();
-      bool isEqualExceptRef = false;
-      nsresult  rv = imageURI->EqualsExceptRef(docURI, &isEqualExceptRef);
-      if (NS_SUCCEEDED(rv) && isEqualExceptRef) {
-        return;
+      if (imageURI) {
+        bool isEqualExceptRef = false;
+        nsresult  rv = imageURI->EqualsExceptRef(docURI, &isEqualExceptRef);
+        if (NS_SUCCEEDED(rv) && isEqualExceptRef) {
+          return;
+        }
       }
     }
 #endif
     aValue.StartImageLoad(aDocument);
     if (aForTokenStream && aContext) {
       CSSVariableImageTable::Add(aContext, aProperty,
                                  aValue.GetImageStructValue());
     }