diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index fcb9410ce03..3ebbc1cd13e 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,9 @@
+2016-12-15  Martin Sebor  <msebor@redhat.com>
+
+	PR bootstrap/78817
+	* vec.h (vec<T, va_heap, vl_ptr>::safe_grow_cleared): Assert
+	a pointer is non-null.
+
 2016-12-15  Andrew Senkevich  <andrew.senkevich@intel.com>
 
 	* config/i386/avx512bwintrin.h: Add new k-mask intrinsics.
diff --git a/gcc/vec.h b/gcc/vec.h
index aa93411132b..3877f2d8ffe 100644
--- a/gcc/vec.h
+++ b/gcc/vec.h
@@ -1607,10 +1607,16 @@ inline void
 vec<T, va_heap, vl_ptr>::safe_grow_cleared (unsigned len MEM_STAT_DECL)
 {
   unsigned oldlen = length ();
-  size_t sz = sizeof (T) * (len - oldlen);
-  safe_grow (len PASS_MEM_STAT);
-  if (sz != 0)
-    memset (&(address ()[oldlen]), 0, sz);
+  gcc_checking_assert (oldlen <= len);
+
+  if (size_t sz = sizeof (T) * (len - oldlen))
+    {
+      safe_grow (len PASS_MEM_STAT);
+
+      T *p = address ();
+      gcc_assert (p != NULL);
+      memset (p + oldlen, 0, sz);
+    }
 }