diff --git a/gcc/tree-ssa-math-opts.cc b/gcc/tree-ssa-math-opts.cc
index bb67dca560b..cd3fd2fc8fb 100644
--- a/gcc/tree-ssa-math-opts.cc
+++ b/gcc/tree-ssa-math-opts.cc
@@ -2982,6 +2982,11 @@ convert_plusminus_to_widen (gimple_stmt_iterator *gsi, gimple *stmt,
   if (to_mode == from_mode)
     return false;
 
+  /* For fixed point types, the mode classes could be different
+     so reject that case. */
+  if (GET_MODE_CLASS (from_mode) != GET_MODE_CLASS (to_mode))
+    return false;
+
   from_unsigned1 = TYPE_UNSIGNED (type1);
   from_unsigned2 = TYPE_UNSIGNED (type2);
   optype = type1;