diff --git a/xpcom/glue/nsISupportsImpl.cpp b/xpcom/glue/nsISupportsImpl.cpp
index 4f0170c5d65..75198aacce6 100644
--- a/xpcom/glue/nsISupportsImpl.cpp
+++ b/xpcom/glue/nsISupportsImpl.cpp
@@ -8,7 +8,7 @@ nsresult NS_FASTCALL
 NS_TableDrivenQI(void* aThis, const QITableEntry* entries,
                  REFNSIID aIID, void **aInstancePtr)
 {
-  while (entries->iid) {
+  do {
     if (aIID.Equals(*entries->iid)) {
       nsISupports* r =
         reinterpret_cast<nsISupports*>
@@ -19,7 +19,7 @@ NS_TableDrivenQI(void* aThis, const QITableEntry* entries,
     }
 
     ++entries;
-  }
+  } while (entries->iid);
 
   *aInstancePtr = nullptr;
   return NS_ERROR_NO_INTERFACE;
diff --git a/xpcom/glue/nsISupportsImpl.h b/xpcom/glue/nsISupportsImpl.h
index bd09a65f171..27abc32a1cb 100644
--- a/xpcom/glue/nsISupportsImpl.h
+++ b/xpcom/glue/nsISupportsImpl.h
@@ -609,8 +609,15 @@ NS_IMETHODIMP _class::QueryInterface(REFNSIID aIID, void** aInstancePtr)      \
                reinterpret_cast<char*>((_class*) 0x1000))                     \
   },
 
+/*
+ * XXX: we want to use mozilla::ArrayLength (or equivalent,
+ * MOZ_ARRAY_LENGTH) in this condition, but some versions of GCC don't
+ * see that the static_assert condition is actually constant in those
+ * cases, even with constexpr support (?).
+ */
 #define NS_INTERFACE_TABLE_END_WITH_PTR(_ptr)                                 \
   { nullptr, 0 } };                                                           \
+  static_assert((sizeof(table)/sizeof(table[0])) > 1, "need at least 1 interface"); \
   rv = NS_TableDrivenQI(static_cast<void*>(_ptr),                             \
                         table, aIID, aInstancePtr);