diff --git a/js/xpconnect/src/XPCQuickStubs.h b/js/xpconnect/src/XPCQuickStubs.h
index dd431a043858..2086cb1c9fd3 100644
--- a/js/xpconnect/src/XPCQuickStubs.h
+++ b/js/xpconnect/src/XPCQuickStubs.h
@@ -49,7 +49,7 @@
 
 class XPCCallContext;
 
-#define XPC_QS_NULL_INDEX  ((size_t) -1)
+#define XPC_QS_NULL_INDEX  ((uint16_t) -1)
 
 struct xpc_qsPropertySpec {
     const char *name;
@@ -76,8 +76,8 @@ struct xpc_qsHashEntry {
     const xpc_qsFunctionSpec *functions;
     // These last two fields index to other entries in the same table.
     // XPC_QS_NULL_ENTRY indicates there are no more entries in the chain.
-    size_t parentInterface;
-    size_t chain;
+    uint16_t parentInterface;
+    uint16_t chain;
 };
 
 inline nsISupports*
diff --git a/js/xpconnect/src/qsgen.py b/js/xpconnect/src/qsgen.py
index a0592e980d86..f711a6f014e8 100644
--- a/js/xpconnect/src/qsgen.py
+++ b/js/xpconnect/src/qsgen.py
@@ -1235,6 +1235,9 @@ def writeDefiner(f, conf, interfaces):
     f.write("static const xpc_qsHashEntry tableData[] = {\n")
     f.write(",\n".join(entries))
     f.write("\n    };\n\n")
+    f.write("// Make sure our table indices aren't overflowed\n"
+            "PR_STATIC_ASSERT((sizeof(tableData) / sizeof(tableData[0])) < (1 << (8 * sizeof(tableData[0].parentInterface))));\n"
+            "PR_STATIC_ASSERT((sizeof(tableData) / sizeof(tableData[0])) < (1 << (8 * sizeof(tableData[0].chain))));\n\n")
 
     # the definer function (entry point to this quick stubs file)
     f.write("JSBool %s_DefineQuickStubs(" % conf.name)