diff --git a/docshell/base/nsDocShell.cpp b/docshell/base/nsDocShell.cpp
index dd54d88d6e91..5577d3a5a104 100644
--- a/docshell/base/nsDocShell.cpp
+++ b/docshell/base/nsDocShell.cpp
@@ -8647,8 +8647,7 @@ nsDocShell::CreateContentViewer(const nsACString& aContentType,
     // Be sure to have a correct mLSHE, it may have been cleared by
     // EndPageLoad. See bug 302115.
     if (mSessionHistory && !mLSHE) {
-      int32_t idx;
-      mSessionHistory->LegacySHistory()->GetRequestedIndex(&idx);
+      int32_t idx = mSessionHistory->LegacySHistory()->GetRequestedIndex();
       if (idx == -1) {
         idx = mSessionHistory->Index();
       }
@@ -11567,8 +11566,7 @@ nsDocShell::OnNewURI(nsIURI* aURI, nsIChannel* aChannel,
   } else if (mSessionHistory && mLSHE && mURIResultedInDocument) {
     // Even if we don't add anything to SHistory, ensure the current index
     // points to the same SHEntry as our mLSHE.
-    int32_t index = 0;
-    mSessionHistory->LegacySHistory()->GetRequestedIndex(&index);
+    int32_t index = mSessionHistory->LegacySHistory()->GetRequestedIndex();
     if (index == -1) {
       index = mSessionHistory->Index();
     }
@@ -12238,8 +12236,7 @@ nsDocShell::AddToSessionHistory(nsIURI* aURI, nsIChannel* aChannel,
       // Replace current entry in session history; If the requested index is
       // valid, it indicates the loading was triggered by a history load, and
       // we should replace the entry at requested index instead.
-      int32_t index = 0;
-      mSessionHistory->LegacySHistory()->GetRequestedIndex(&index);
+      int32_t index = mSessionHistory->LegacySHistory()->GetRequestedIndex();
       if (index == -1) {
         index = mSessionHistory->Index();
       }
diff --git a/docshell/shistory/ChildSHistory.cpp b/docshell/shistory/ChildSHistory.cpp
index c0321e6199ac..0c6b40cb1133 100644
--- a/docshell/shistory/ChildSHistory.cpp
+++ b/docshell/shistory/ChildSHistory.cpp
@@ -32,9 +32,7 @@ ChildSHistory::~ChildSHistory()
 int32_t
 ChildSHistory::Count()
 {
-  int32_t count;
-  mHistory->GetCount(&count);
-  return count;
+  return mHistory->GetCount();
 }
 
 int32_t
diff --git a/docshell/shistory/nsISHistory.idl b/docshell/shistory/nsISHistory.idl
index 7daef0a86b43..1159ac2d3419 100644
--- a/docshell/shistory/nsISHistory.idl
+++ b/docshell/shistory/nsISHistory.idl
@@ -28,7 +28,7 @@ interface nsIURI;
  * This interface is accessible from javascript.
  */
 
-[scriptable, uuid(7b807041-e60a-4384-935f-af3061d8b815)]
+[builtinclass, scriptable, uuid(7b807041-e60a-4384-935f-af3061d8b815)]
 interface nsISHistory: nsISupports
 {
   /**
@@ -45,10 +45,11 @@ interface nsISHistory: nsISupports
    * the number of toplevel documents currently available
    * in session history.
    */
-  readonly attribute long count;
+  [infallible] readonly attribute long count;
 
   /**
-   * The index of the current document in session history.
+   * The index of the current document in session history. Not infallible
+   * because setting can fail if the assigned value is out of range.
    */
   attribute long index;
 
@@ -58,7 +59,7 @@ interface nsISHistory: nsISupports
    * didn't finished yet. When document finishes the loading
    * value -1 is returned.
    */
-  readonly attribute long requestedIndex;
+  [infallible] readonly attribute long requestedIndex;
 
   /**
    * Get the history entry at a given index. Returns non-null on success.
diff --git a/docshell/shistory/nsSHistory.cpp b/docshell/shistory/nsSHistory.cpp
index 944eb896fe74..2239d8908d25 100644
--- a/docshell/shistory/nsSHistory.cpp
+++ b/docshell/shistory/nsSHistory.cpp
@@ -653,7 +653,7 @@ nsSHistory::AddEntry(nsISHEntry* aSHEntry, bool aPersist)
 NS_IMETHODIMP
 nsSHistory::GetCount(int32_t* aResult)
 {
-  NS_ENSURE_ARG_POINTER(aResult);
+  MOZ_ASSERT(aResult, "null out param?");
   *aResult = Length();
   return NS_OK;
 }