diff --git a/go/expressions.cc b/go/expressions.cc
index 2c1286c..929571b 100644
--- a/go/expressions.cc
+++ b/go/expressions.cc
@@ -12158,6 +12158,13 @@
       return Expression::make_error(loc);
     }
 
+  // Avoid copy for string([]byte) conversions used in map keys.
+  // mapaccess doesn't keep the reference, so this is safe.
+  Type_conversion_expression* ce = this->index_->conversion_expression();
+  if (ce != NULL && ce->type()->is_string_type()
+      && ce->expr()->type()->is_slice_type())
+    ce->set_no_copy(true);
+
   if (!Type::are_identical(mt->key_type(), this->index_->type(),
 			   Type::COMPARE_ERRORS | Type::COMPARE_TAGS,
 			   NULL))
diff --git a/go/statements.cc b/go/statements.cc
index 2e2d039..67ab184 100644
--- a/go/statements.cc
+++ b/go/statements.cc
@@ -1307,6 +1307,13 @@
   if (map_type == NULL)
     return Statement::make_error_statement(loc);
 
+  // Avoid copy for string([]byte) conversions used in map keys.
+  // mapaccess doesn't keep the reference, so this is safe.
+  Type_conversion_expression* ce = map_index->index()->conversion_expression();
+  if (ce != NULL && ce->type()->is_string_type()
+      && ce->expr()->type()->is_slice_type())
+    ce->set_no_copy(true);
+
   Block* b = new Block(enclosing, loc);
 
   // Move out any subexpressions to make sure that functions are